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/search.md

@@ -113,8 +113,6 @@ Will make <kbd>Ctrl</kbd> + <kbd>K</kbd> focus the search bar for Windows users
 Sometimes you might have a page that you don't want to be indexed for the search nor to show up in search results, e.g., a 404 page.
 To exclude a page from search, add the `search_exclude: true` parameter to the page's YAML front matter:
 
-#### Example
-
 {: .no_toc }
 
 ```yaml
@@ -158,7 +156,8 @@ By default, the search feature indexes a page's `.content`, `.title`, and *some*
 2. Add a new file named `_includes/lunr/custom-data.json`. Insert custom Liquid code that reads your data (e.g. the page object at `include.page`) then generates custom Javascript fields that hold the custom data you want to index. Verify these fields in the generated `assets/js/search-data.json`.
 3. Add a new file named `_includes/lunr/custom-index.js`. Insert custom Javascript code that reads your custom Javascript fields and inserts them into the search index. You may want to inspect `assets/js/just-the-docs.js` to better understand the code.
 
-#### Example
+### Example: adding custom `usage` and `examples` fields
+{: .text-delta }
 
 This example adds front matter `usage` and `examples` fields to the search index.