docs: fix (non-systemic) accessibility issues flagged by aXe (#1531)

This PR fixes some accessibility issues in our theme docs (i.e. not generated code) flagged by #1513. Here, I target changes that I say are not "systemic", i.e. issues that are easily resolvable by changing our copy and page structure (rather than issues that are created by how kramdown/rouge generates HTML, or reworking our color themes).

Here's a quick summary of the manual changes I made:

- ~~writing some JS to set `tabindex="0"` on all code blocks; I'd prefer a ruby-native solution, but that involves writing Ruby code, which is incompatible with the pages gem~~ I've moved this to #1533
- rewriting many headings named "Example" which were almost always h4s into more descriptive headings + the appropriate heading level, adding .text-delta to maintain the previous style when necessary
- removing some old heading ID hacks in `index-test` that are no longer necessary, since Jekyll does this automatically now
- fixing the table headings in `docs/utilities/layout.md`
- adding accessible titles + descriptions to the mermaid examples
- occasionally, slightly moving around copy to make it align with new headings

If you test with #1513 with the following rules disabled:

```rb
skipped_rules = [
  'color-contrast', # requires theme auditing
  # issues w/ autogenerated footnotes
  'aria-allowed-role',
  'landmark-no-duplicate-contentinfo',
  'landmark-unique',
  'aria-deprecated-role',
  # issues w/ markdown checkboxes
  'label'
]
```

You should get passing tests :) which is awesome!

## next steps

1. we need to do a pass over our docs copy - very inconsistent. This has been a pain point for me for a while now, just need to find time to sit down and do it. In particular, I'd love to standardize how we display example code (perhaps even hiding it with `<details>` and `<summary>`?), our headings language, what goes into the ToC, our overall writing style, etc.
2. ~~I don't love the JS hack for adding `tabindex="0"` to code blocks (so that they are keyboard-focusable). Ideally, we'd add a custom formatter to rouge to do this, but we can't execute arbitrary Ruby code when users use `github-pages`. I'll look into this some more - maybe rouge would be open to adding this as a feature.~~ see: #1533
4. There are some systemic issues that need a deeper look:
    1. The most common issue is still color-contrast. Fixing this involves:
        - looking at our whites/blacks/grays for core text and highlighting
        - reevaluating our syntax highlighting themes
        - fixing dark mode, once and for all :) 
        - also, picking accessible callout colours!
     2. kramdown's autogenerated footnotes feature creates a bunch of errors that aXe flags: it seems like a deprecated aria role is being used, and perhaps some misuse of markup. Need to look into this more before I can make a solid attempt at resolving this issue.
     3. We demonstrate the use of `- [ ]`, but this generates `<input type="checkbox">` values with no label. I'm not entirely sure what the best way to fix this problem is (without writing custom Ruby code). I'll have to think about this some more.

---------

Co-authored-by: Michael Ball <michael@mball.co>

docs/utilities/layout.md

@@ -18,7 +18,7 @@ parent: Utilities
 
 These spacers are available to use for margins and padding with responsive utility classes. Combine these prefixes with a screen size and spacing scale to use them responsively.
 
-| Classname prefix | What it does                  |
+| Classname prefix | Related CSS Property          |
 |:-----------------|:------------------------------|
 | `.m-`            | `margin`                      |
 | `.mx-`           | `margin-left`, `margin-right` |
@@ -28,7 +28,7 @@ These spacers are available to use for margins and padding with responsive utili
 | `.mb-`           | `margin-bottom`               |
 | `.ml-`           | `margin-left`                 |
 
-| Classname prefix | What it does                    |
+| Classname prefix | Related CSS Property            |
 |:-----------------|:--------------------------------|
 | `.p-`            | `padding`                       |
 | `.px-`           | `padding-left`, `padding-right` |
@@ -54,8 +54,8 @@ Spacing values are based on a `1rem = 16px` spacing scale, broken down into thes
 
 Use `mx-auto` to horizontally center elements.
 
-#### Examples
-{: .no_toc }
+### Applying Spacing Utilities with `{: }`
+{: .no_toc .text-delta }
 
 In Markdown, use the `{: }` wrapper to apply custom classes:
 
@@ -69,7 +69,7 @@ This paragraph will have 2rem/32px of padding on the right and left at all scree
 
 ## Horizontal Alignment
 
-| Classname               | What it does                     |
+| CSS Class               | Applied CSS Declaration          |
 |:------------------------|:---------------------------------|
 | `.float-left`           | `float: left`                    |
 | `.float-right`          | `float: right`                   |
@@ -82,7 +82,7 @@ _Note: any of the `flex-` classes must be used on a parent element that has `d-f
 
 ## Vertical Alignment
 
-| Classname              | What it does                    |
+| CSS Class              | Applied CSS Declaration         |
 |:-----------------------|:--------------------------------|
 | `.v-align-baseline`    | `vertical-align: baseline`      |
 | `.v-align-bottom`      | `vertical-align: bottom`        |
@@ -95,7 +95,7 @@ _Note: any of the `flex-` classes must be used on a parent element that has `d-f
 
 Display classes aid in adapting the layout of the elements on a page:
 
-| Class             |                         |
+| CSS Class         | Applied CSS Declaration |
 |:------------------|:------------------------|
 | `.d-block`        | `display: block`        |
 | `.d-flex`         | `display: flex`         |
@@ -105,8 +105,8 @@ Display classes aid in adapting the layout of the elements on a page:
 
 Use these classes in conjunction with the responsive modifiers.
 
-#### Examples
-{: .no_toc }
+### Applying Display Utilities with `{: }`
+{: .no_toc .text-delta }
 
 In Markdown, use the `{: }` wrapper to apply custom classes: