feat: mobile/desktop docs

[SRWieZ]

[simonhamp]

I like the switcher. It may need to be simplified when viewing on mobile, but seems fine for now

My only challenge is with the URLs

[SRWieZ]

Modified as per review.

Also:

• Emptied the boilerplate of mobile documentation.
• Created a new variable $platform everywhere $version was available.
• Fixed a width issue when there was no text on a page (visible in screenshot above).
• Updated URLs from docs/1/ to docs/desktop/1/ in *.md files.
• Put a 302 redirect on old URLs (you can change it to 301 when you want; it's just that 301 cache is so hard to clear, I preferred to wait for the full green light).
• Update SEO title to include platform and version

[SRWieZ]

Reverted the following fix that made it worse in large screen:

• Fixed a width issue when there was no text on a page (visible in screenshot above).