Replace javascript highlighting using Prism with the Rouge markdown
highlighter. Uses the github theme.

Add better responsive CSS for tables in guides

Fix borders & breakpoints for responsive tables

Add table fixes to RTL stylesheets

Use 480px breakpoints, remove outdated CSS

Follow-up to #39594, which added CSS in order to select shell commands
sans prompts on triple-click.

This commit adds several bash code fences and prompts where they were
missing, and removes a few where they were inappropriate.

Shell prompt is already unselectable
Co-authored-by: NJonathan Hefner <jonathan@hefner.pro>

I used a simple HTML-and-CSS-based approach to achieve the look of the edge badge.

* Delete edge_badge.png
* Update layout.html.erb
* Update main.css

When line-height has unit, it will have unexpected results.
Based on recommendations by MDN[1] and CSSTricks[2].

[1] MDN https://developer.mozilla.org/en-US/docs/Web/CSS/line-height#Prefer_unitless_numbers_for_line-height_values
[2] CSS Tricks https://css-tricks.com/almanac/properties/l/line-height/#article-header-id-0

The previous version wasn’t matching the Rails Guides logo source file
listed at https://github.com/rails/homepage/blob/master/materials/rails_guides_logo.psd
somehow. It was also a GIF when an 8-bit PNG results in a smaller file (2KB vs 4KB) which
should be just as widely compatible with browsers.

And that’s just for the 1x version. I added a new 2x version for “retina” or high-density
displays and a CSS @media query to progressively enhance with this high density
version of the logo. I’ve ensured that this query is supported across browsers by
using vendor-specific versions when necessary. See:
https://caniuse.com/#feat=css-media-resolution

[ci skip]

This notably allows us to tile each category without pushing the page down vertically
as much as we previously did on any viewport wider than mobile sizes.

It also means we can fit more guides which will become useful in the future since
we have several new guides in the work.

Finally the new layout allows three clearly distinct columns to emerge:
- Start Here and Rails basics: Models, Views, Controllers, Other Components
- Digging Deeper, Extending Rails, Contributing, Maintenance
- Release Notes

Having Release Notes shoot back up to the top of the third column is great
because that makes finding the latest released version and its associated
release notes much easier without having to scroll down a bunch.

[ci skip]

Fix a layout issue in the rails guides, where the navigation covers the main text,
if the page is between 800 and 960 pixels wide. (issue #33406)

* Rewrite with Vanilla JS
* Confirmed with Chrome, Safari, Firefox

* .clearfix is overridden.
* .clear is not currently used.
* #extraCol is not currently used.
* table th, table td are overridden. Merged them.
* Removed needless comment lines which are added on #6475

Minor fix to #28662

[ci skip]

To allow easy linking at all levels and not just from index

Selectively add `li ul, li ol { margin:0 1.5em; }` to subColl needed for Chapters column, instead of leaking to other lists.

Before:
 https://monosnap.com/file/CNIMXcqv3HuRrObTNYWYFis8I3G8PT
After:
 https://monosnap.com/file/0IyEKEy9hcRCIx82etR8ikinCo3vNb

Chapters column remains as is: http://take.ms/M0BkF

Commit 65a2977c added a `pre-wrap` style for white spaces on `.note`
paragraphs. However, this is first inconsistent as other notes like
warnings don't have this style applied. Furthermore, it seems to be
unneeded for mobile devices.

Also revert changes made in #18147 since they aren't needed anymore.

Cross-refs #18138.

[ci skip]

dropdown menu on guides page (which appear on small windows)
[ci skip]

Commit 65a2977c has introduced some
changes to format content for small devices, this change reverts the
original font size for normal screens while maintaing the same format
for these devices.

[ci skip]

Fix some indenting for some properties and closing }, remove tabs, and
merge two pre/code definitions.

[ci skip]

Current stable guides show the "Guides Index" links without underline,
just showing it when it's hovered. This restores the same functionality
for edge guides.

Also remove annoying pointer cursor while accessing guides index,
restoring previously functionality: now the pointer cursor only appears
when hovering a link.

Closes #8107 [ci skip]

This reverts commit 7f536f91.

Reason: If at all this is done, this should be done in rails master.