Reorg CE Docs landing page

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31069

Reorg CE Docs landing page in more logical topics.

Screenshot

• 1st version
• 2nd version

Current version:

Thinking this through

In the future, I can picture the headings with FontAwesome icons by their side. Also, we can make the headings accordions if we want.

Btw, by organizing docs this way it will be clearer to identify what's missing and what needs update.

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@SeanPackham can you please take a look to check if you want me to go in this direction? Thanks ;) cc/ @axil