docs: switch to new Lit template

[Borda]

What does this PR do?

apart from the new template is also:

• stops publish docs on RTFD (only for PR will do it)
• build artifact with fetches assets only for deployment not on PR

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?

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 🙃

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📚 Documentation preview 📚: https://torchmetrics--2097.org.readthedocs.build/en/2097/

[Borda]

hmm, the broken MD links seem to be unrelated... :/ #2098

[codecov]

Codecov Report

Merging #2097 (dacfba0) into master (809bb5f) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2097   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         293     293           
  Lines       16299   16299           
======================================
  Hits        14177   14177           
  Misses       2122    2122