Enable Nitpicky mode in Sphinx docs

[l1storez]

What do these changes do?

Make sphinx strict about any warnings.

Are there changes in behavior for the user?

No

Related issue number

Resolves #5518

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov]

Codecov Report

Merging #5774 (9535ae6) into master (4af3d4a) will decrease coverage by 0.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5774      +/-   ##
==========================================
- Coverage   97.17%   96.51%   -0.67%     
==========================================
  Files          41       41              
  Lines        8865     8832      -33     
  Branches     1425     1416       -9     
==========================================
- Hits         8615     8524      -91     
- Misses        133      179      +46     
- Partials      117      129      +12     

Flag	Coverage Δ
unit	96.51% <ø> (-0.56%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/web_runner.py	92.34% <0.00%> (-5.41%)	⬇️
aiohttp/tcp_helpers.py	94.73% <0.00%> (-5.27%)	⬇️
aiohttp/pytest_plugin.py	94.03% <0.00%> (-3.45%)	⬇️
aiohttp/connector.py	94.61% <0.00%> (-1.91%)	⬇️
aiohttp/test_utils.py	97.78% <0.00%> (-1.90%)	⬇️
aiohttp/http_writer.py	97.36% <0.00%> (-1.76%)	⬇️
aiohttp/web.py	97.43% <0.00%> (-1.71%)	⬇️
aiohttp/http_parser.py	96.19% <0.00%> (-1.27%)	⬇️
aiohttp/web_protocol.py	85.50% <0.00%> (-0.91%)	⬇️
aiohttp/http_websocket.py	97.84% <0.00%> (-0.81%)	⬇️
... and 6 more

---

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[patchback]

Backport to 3.8: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.8/87e4f3b2270f81ee0831005d98d52301e8ecfbe9/pr-5774

Backported as #5780

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