Documentation landing page

[nijel]

Describe the problem

The documentation landing page at https://docs.weblate.org/en/latest/ should make it easier for users to navigate. Before 0804b38 we had a README as shown at GitHub, but that really doesn't fit, so I've replaced it with a basic landing page to extend it.

Describe the solution you would like

The typical landing page seems to list the most frequently needed guides. We might figure out that we miss some of them, but let's start with listing the ones we have, we can add more later.

Describe alternatives you have considered

No response

Screenshots

No response

Additional context

Some examples:

• https://docs.gitlab.com/
• https://www.passbolt.com/docs/
• https://docs.gitlab.com/