Improve layout of documentation page and add search feature (#8247)

* Improve layout of documentation page and add search feature

Closes https://github.com/FreshRSS/FreshRSS/issues/7915, https://github.com/FreshRSS/FreshRSS/issues/5325

Also: anchor headings and fix building site locally

* Further improvements

* Set color of hyperlinks
* Consistent styling of close aside button across devices
* Mobile layout 600px -> 1200px
* Add suffix to docs `<title>`
    * Note: titles of pages probably need to be improved, since currently they are just derived from the names of the first heading on every page
* Add favicon

* Improve font

* Try to fix favicon not loading correctly on GH pages

* Use local font

* Attempt to fix GH pages

* Final improvements

* Copy to clipboard button
* Support for nojs search
* Dark mode
* Load search.json (200KB json) only on search input focus
* Keep scroll state of sidebar across navigations

* Clickable images and CSP

CSP so we avoid hotlinking resources and clickable images are useful for zooming on mobile for example

* Fix typos

* Disable Dark Reader extension if dark mode CSS is loaded

* Support internationalisation (via language dropdown)

* Add Gemfile.lock

* Make CI build work with the custom plugin

* Make menus closable with Esc

* Fix typos CI

* Suggestions

* Use `ruby/setup-ruby` action in workflow for installing and caching gems.

* Run build only when there are changes to `docs/`

See: https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows?versionId=free-pro-team%40latest&productId=actions#running-your-workflow-only-when-a-push-to-specific-branches-occurs

* Change font to `Open Sans`

* Increase line height

* Fix Liquid syntax error

1 files changed, 282 insertions, 0 deletions

diff --git a/docs/assets/css/highlight.css b/docs/assets/css/highlight.css
new file mode 100644
index 000000000..6b5c4b725
--- /dev/null
+++ b/docs/assets/css/highlight.css
@@ -0,0 +1,282 @@
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