docs: initialize Sphinx Gallery

[Borda]

What does this PR do?

Init for #2438
references:

• https://sphinx-gallery.github.io/stable/getting_started.html
• https://stackoverflow.com/a/57216937

Remaining todo:

• make thumbnail larger -> another PR
• skip gallery for fast docs builds
• add extra job/check for gallery

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 🙃

---

📚 Documentation preview 📚: https://torchmetrics--2508.org.readthedocs.build/en/2508/

[Borda]

asking about supporting ! pip install ... in sphinx-gallery/sphinx-gallery#129

[Borda]

sample: https://torchmetrics--2508.org.readthedocs.build/en/2508/gallery/image/spatial_correlation_coef.html

just the div is too small

ref: https://sphinx-gallery.github.io/stable/configuration.html#setting-gallery-thumbnail-size

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69%. Comparing base (f6d1f44) to head (6a57e5a).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2508   +/-   ##
======================================
  Coverage      69%     69%           
======================================
  Files         313     313           
  Lines       17617   17617           
======================================
  Hits        12163   12163           
  Misses       5454    5454           

[stancld]

Looks cool ❤️