Revamp homepage of Developer Resources to match other sites

[ocean90]

Compare ​https://developer.wordpress.org/ with ​https://wordpress.org/support/ or ​https://wordpress.org/plugins-wp/.

[samuelsidler]

@coffee2code was working on a revamp of the devhub homepage with the correct (new) colors and refreshed design.

[SergeyBiryukov]

meta-2459.patch​ changes the background: meta-2459.PNG​.

[coffee2code]

In 4985:

developer.wordpress.org: Update front page blue background to match blue used in newer w.org themes.

A minor tweak until the theme sees a more substantial update to align with newer theme styling.

Props SergeyBiryukov.
See #2459.

[melchoyce]

This could use some additional design improvements. Tagging to solicit design feedback and suggestions.

[coffee2code]

In 6512:

developer.wordpress.org: Adapt to new handbook styles as introduced in the Make team sites.

• Revamps style of handbook sidebar (including making menu collapsible) and in-content table of contents
• Removes search pane (with imminent switch to search widget) for handbooks
• Moves breadcrumbs into main content area for handbooks
• Moves #masthead out of, and before, #page
• Restyles masthead to have blue background
• Restyles slide down search pane to not match new masthead
• Introduces dropdown menu for masthead menu at smaller viewports (as used in WP-CLI Commands)
• Minor front page revamp of colors (more coming)

Fixes #2969,
See #2459, #3377.

[coffee2code]

In 7739:

developer.wordpress.org: Revamp the design of the homepage to match the redesign of other wp.org sites (support, plugins, main, etc).

Also changes dashicon used for theme handbook.

Props mapk, coffee2code.
Fixes #2459.
See #3377.