Better link between website and docsite (and github)

[june-andronick]

(This may wait for the Information Architecture feedback first)

The links between the seL4 website and the docsite (and github) should be improved.

Currently:

• finding a link to the docsite from the website is difficult. It should be obvious. And it should be obvious that both co-exist
• we need to think which important pages form docsite should be made visible and easily found on the website (e.g. FAQ, roadmap, projects...)
• some topic overlap should be discuss, eg
  • TSC is on the website, roles description is on the docsite
  • Learn and Contribute have a landing pages on the website, with various links to the docsite; we should think how much information should be on both, or docsite should link back to website.