Fix: improve build time (#1358)

* Remove "passive" toggle

PR #1244 introduced the "passive" toggle, but just-the-docs.js subsequently disabled the only styling that used it, so it became redundant.
This removes it.

* Reduce build time for page-dependent CSS

Fix #1323

- Remove `_includes/head_nav.html`.
- Generate page-independent SCSS in `assets/css/just-the-docs-head-nav.css`.
- Link to `/assets/css/just-the-docs-head-nav.css` in `head.html`.
- Disable the above stylesheet in `assets/js/just-the-docs.js`.
- Generate page-dependent CSS in `_includes/css/activation.scss.liquid` and include in `head.html`.

* No override svg rotate

* Disable both stylesheets safely

* Move the site nav to a new include

- Fix the complete site nav
- Move the site nav to `_includes/site_nav.html`
- Cache the site nav
- Uncache `nav.html`

* Move nav and site_nav to _includes/components

* Replace id prefix

* Update breadcrumbs.html

Replace several filters by a single loop through all the pages,
but breaking as soon as possible.

Profiling indicates that this saves up to 50% of the breadcrumbs build time for the filters.

* Update just-the-docs-head-nav.css

Adjust the number of lines to keep

* Update head.html

Remove superflous type.

* Update activation.scss.liquid

Remove a superfluous closing brace.
Adjust layout.

* Use `scssify` to remove nesting

Preliminary profiling indicates that using `scssify` on the small number of nested CSS rules produced by `activation.scss.liquid` is quick enough.

* Update head.scss

Manual attempt at prettier (pending installation in Atom).

* Avoid generation of nested CSS

Local profiling indicated that using `scssify` on each page takes about 1% of the build time.

- Update `_includes/css/activation.scss.liquid` to generate non-nested CSS.
- Remove use of `scssify` from `_includes/head.html`.

* Ignore false positives from validator

Ignores: `:1.810-1.823: error: CSS: Parse Error.` and `:1.811-1.824: error: CSS: Parse Error.`; had to shift things around since the local config overrides the CI flag.

* Inline `_sass/head.css`

* Update CHANGELOG.md

---------

Co-authored-by: Matthew Wang <matt@matthewwang.me>

CHANGELOG.md

@@ -20,8 +20,14 @@ Code changes to `main` that are *not* in the latest release:
 ### Bugfixes
 
 - Fixed: remove href from the navigation link to the current page by [@pdmosses] in [#1356]
+- Fixed: improve build time by [@pdmosses] in [#1358]
+
+{: .warning }
+[#1358] moved `_includes/nav.html` to the `_includes/components` directory,
+Users who were overriding that file will need to adjust their sites accordingly.
 
 [#1356]: https://github.com/just-the-docs/just-the-docs/pull/1356
+[#1358]: https://github.com/just-the-docs/just-the-docs/pull/1358
 
 ## Release v0.6.2