docs: enable nitpicky mode to warn on broken references

[krusta07]

Summary of Changes

Enabled nitpicky = True in docs/conf.py so Sphinx emits
warnings on broken references during documentation builds.

Also added nitpick_ignore entries for the three unresolved
type aliases (SyncMiddleware, AsyncMiddleware,
PreparedMiddlewareResult) that were already noted in a
TODO comment in the same file.

Related Issues

Closes #1888

Pull Request Checklist

• Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• Added tests for changed code.
• Performed automated tests and code quality checks by running tox.
• Prefixed code comments with GitHub nick and an appropriate prefix.
• Coding style is consistent with the rest of the framework.
• Updated documentation for changed code.
  • Added docstrings for any new classes, functions, or modules.
  • Updated docstrings for any modifications to existing code.
  • Updated both WSGI and ASGI docs (where applicable).
  • Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • Updated all relevant supporting documentation files under docs/.
  • A copyright notice is included at the top of any new modules.
  • Changed/added classes/methods/functions have appropriate directives.
• Changes have towncrier news fragments under docs/_newsfragments/.
• LLM output has been carefully reviewed and tested by a human developer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6a49881) to head (277097e).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2668   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7985      7985           
  Branches      1103      1103           
=========================================
  Hits          7985      7985           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[vytas7]

Hi, and thanks for experimenting with this.
Unfortunately, bluntly enabling nitpicky like this results in hundreds of warnings...

[vytas7]

Please do not impersonate me in the comments, use your own nick in the prefix.