The old `sassc` gem uses libsass which is deprecated, replaced by Dart
sass which is used by sass-embedded and jekyll-sass-converter 3+.

This PR updates to use Dart Sass, to help keep new changes compatible
with future Sass and Jekyll versions but also as a step towards
#4164.

Using `/` as a division operator [is
deprecated](https://sass-lang.com/documentation/breaking-changes/slash-div),
so the options are `calc(x / y)`, `math.div(x, y)`, or in some cases it
makes sense to switch to multiplication instead. This PR chooses
switching to multiplication for most instances.

- Removes unused styles
- Improves formatting and spacing
- Moves some code to proper locations
- Removes now unnecessary files

Preparation to make future work towards
#4164 easier. Has the added
benefit of reducing final CSS size a bit.

`@import` is being deprecated in favor of `@use` in Dart Sass (which we
are now using). Read more within Sass's [`@import`
documentation](https://sass-lang.com/documentation/at-rules/import).

Contributes to #4164 since
it will make that migration easier, with all bootstrap usages
namespaced. Also enables better code analysis and auto-completion in
IDEs.

- Removes unused styles
- Improves formatting and spacing
- Moves some code to proper locations
- Removes now unnecessary files

Preparation to make future work towards
dart-lang#4164 easier. Has the added
benefit of reducing final CSS size a bit.

…-lang#4974)

`@import` is being deprecated in favor of `@use` in Dart Sass (which we
are now using). Read more within Sass's [`@import`
documentation](https://sass-lang.com/documentation/at-rules/import).

Contributes to dart-lang#4164 since
it will make that migration easier, with all bootstrap usages
namespaced. Also enables better code analysis and auto-completion in
IDEs.

- Updates the sidenav static generation logic to match `flutter/website`
for improved build performance and consistency. Also removes the need
for specifying `match-page-url-exactly` on directory index pages.
- This changes the 11ty related `filters.ts` and `eleventyComputed.js`
files. These changes have already been reviewed and been in use by
`flutter/website` for a while.
- Redesigns the sidenav for improved accessibility and reducing
dependence on Bootstrap.
- Reimplements the site layout in preparation for Bootstrap removal and
improving scrolling.
- Removes a bunch of now unneeded logic and styles, further supporting
the later removal of Bootstrap.

Fixes #5791
Fixes #5789
Contributes to #4164
Contributes to #3849
Prepares for #2625

Contributes to #4164
Contributes to #5251

This PR eliminates the usages of bootstrap by styles added for Effective
Dart. Also completes some nearby/related cleanup. No (large) functional
change.

Contributes to #4164

- Removes reliance on bootstrap classes for layout and responsiveness.
- Stops using bootstrap-provided mixins and other utilities.
- Remove some unnecessary div nesting.
- Rely on same elements for narrow and wide layouts so that it's not
required to update two locations.
- Adds a missing missing aria attributes where necessary to improve
accessibility.
- Other general cleanup.

Contributes to #4164
Contributes to #5251

In replacing some of the used bootstrap styles and layouts, this PR also
updates the styles of a few affected components to improve visual
accessibility.

Closes #4164 🚀