Fix ARIA labels for all anchors with href="#"; adds aria-pressed information for toggles

[mattxwang]

This follows up from #1259 and closes #1261. Basically, this PR accomplishes the two items discussed in the issue:

1. for all anchors that are actually buttons (i.e., have href="#"), I've replaced them with a semantic <button>
   • under the hood, I've made a .btn-reset class pulling out the reset from Add ARIA roles and labels to search, header, logo, mobile menu button, and main content #1259, so there's no visual change
2. for anchors that are "toggle buttons" (the mobile menu nav, sidebar children/grandchildren toggles), I've added an aria-pressed property that is updated as the button is clicked

I've also slightly modified some of the aria-labels to make them more consistent. Observe that we shouldn't update these as the button is clicked; screen readers use the aria-pressed property to add an annotation to each button.

To test this,

• the sidebar children and grandchildren can be done on the deploy preview:
  • open an arbitrary page; observe that the sidebar children/grandchildren dropdown ticks now have a proper aria-label and aria-pressed, as well as otherwise work as intended
  • toggle one of the buttons; observe the aria-pressed role changing as this is done
  • open a grandchild page; observe that the aria-pressed has a correct default wrt whether or not the page is active
• the mobile menu can be done on the deploy preview; on a smaller viewport, observe the correct aria-pressed
• two features require local changes to test:
  • the site.search.button needs to be enabled in the _config.yml. To test this, locally clone the repo, change the flag, and observe that the button still works as intended + has no visual regressions.
  • the collections feature is a bit more complicated. To test this, locally clone the repo, add an arbitrary collection and changes to _config.yml, and observe the same behaviour for the sidebar children/grandchildren above

[mattxwang]

@just-the-docs/reviewers if I could get a review on this, that would be great; I'll likely cut a patch release after this change!

(you may want to review this with whitespace off, since there are some artifacts from previous PRs. need to eventually get around to re-enabling prettier)