docs: move overview

[Borda]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

will be reflected in Lightning-AI/pytorch-lightning#19248

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #39 (7b98585) into main (3138018) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@        Coverage Diff         @@
##           main   #39   +/-   ##
==================================
  Coverage    86%   86%           
==================================
  Files         3     3           
  Lines       167   167           
==================================
  Hits        144   144           
  Misses       23    23