Rework the theme to match WP Rocket

[MathieuLamiot]

Context
The current theme is the one from developer.wordpress.org. It contains many links to other WordPress.org websites (developer, make, etc), the colours don't match WP Rocket charts, and there are pages/links we don't need.
We should try to make it look more like docs.wp-rocket.me

Expected behavior

• Replace the header with one similar to docs.wp-rocket.me ; named "Developer Documentation"

- Have a footer similar to docs.wp-rocket.me (except the multilingual part): - The content of the main page should provide links to the following pages: http://localhost:8888/reference/functions/, classes, hooks, methods, commands.

• External links to wordpress.org and related must be removed.

• If possible, a search bar similar to the one on docs.wp-rocket.me should allow to browse from the homepage with this URL: http://localhost:8888/?s=the_searched_text

• Colors of the theme should match the ones of docs.wp-rocket.me in general.