- add `sr-only` "Bootstrap", which will be part of the accessible name
- extra `sr-only` text to give some context - that the dropdown is about switching versions
- remove the redundant id/aria-labelledby for the dropdown
- change the control from `<a>` to `<button>`
`aria-haspopup="true"` is really intended to signal that an ARIA `menu` will be opened on activation. as a result, some assistive technologies will announce controls with `aria-haspopup="true"` as a menu or menu item (e.g. JAWS and NVDA).
In addition, `aria-haspopup` seems to trigger a bug in Edge/Narrator where the `aria-expanded` state is not correctly announced at the moment when `aria-haspopup` is present.
This now makes the dropdown button more like a generic disclosure widget control - see also https://www.w3.org/TR/wai-aria-practices-1.2/examples/disclosure/disclosure-navigation.html01
This commit includes all the needed workarounds and most changes from the main branch for everything to work, like:
* removing empty lines in raw HTML that break output
* read .browserslistrc, CSS variables from disk instead of duplicating it
* using Hugo mounts
* using Hugo for the docs CSS/JS
* move ToC Sass code to a separate file while adapting it for Hugo
Thus, this patch makes our npm scripts faster since lint runs on one step and there's no separate docs assets processing.
