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/customization.md

@@ -20,8 +20,8 @@ Just the Docs supports two color schemes: light (default), and dark.
 
 To enable a color scheme, set the `color_scheme` parameter in your site's `_config.yml` file:
 
-#### Example
-{: .no_toc }
+### Example: preview dark color scheme
+{: .no_toc .text-delta }
 
 ```yaml
 # Color scheme supports "light" (default) and "dark"
@@ -76,8 +76,8 @@ Available variables are listed in the [\_variables.scss](https://github.com/just
 
 For example, to change the link color from the purple default to blue, include the following inside your scheme file:
 
-#### Example
-{: .no_toc }
+#### Example: custom link color
+{: .no_toc .text-delta }
 
 ```scss
 $link-color: $blue-000;
@@ -142,10 +142,10 @@ Additionally, you may want to add completely custom CSS specific to your content
 To do this, put your styles in the file `_sass/custom/custom.scss`.
 This will allow for all overrides to be kept in a single file, and for any upstream changes to still be applied.
 
-For example, if you'd like to add your own styles for printing a page, you could add the following styles.
+### Example: custom print styles
+{: .no_toc .text-delta }
 
-#### Example
-{: .no_toc }
+For example, if you'd like to add your own styles for printing a page, you could add the following styles.
 
 ```scss
 // Print-only styles.
@@ -179,7 +179,7 @@ New (v0.4.0)
 
 If the page has any child pages, and `has_toc` is not set to `false`, this content appears as a heading above the [auto-generating list of child pages]({% link docs/navigation/children.md %}) after the page's content.
 
-#### Example
+#### Example: changing TOC heading
 {: .no_toc }
 
 To change the default TOC heading to "Contents", create `_includes/toc_heading_custom.html` and add: