docs(readme): add table of content in readme

[Hugo-Galley]

Related issue

No issues link, just for enhanced readme

What changed

Add table of content in Readme, for simplify navigation

Checklist

• I discussed the approach on the linked issue before starting (or this is a trivial fix: typo, docs, obvious one-liner)
• This PR is focused on a single change
• I ran npm run format in ui/leafwiki-ui (and in e2e if e2e tests changed)
• I updated documentation if needed

[perber]

I like the idea. Initially, I was a bit skeptical because GitHub already has something like this, but I think your solution is better and helps a lot! At least until we have the documentation/handbook. :)