[PR #12133/4ec897bd backport][3.13] Update Gunicorn documentation link

[patchback]

This is a backport of PR #12133 as merged into master (4ec897b).

The previous link referred to a non existing page (404)

What do these changes do?

Fixes a 404 link to the Gunicorn site

Are there changes in behavior for the user?

no

Is it a substantial burden for the maintainers to support this?

no

Related issue number

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_or_pr_num>.<type>.rst (e.g. 588.bugfix.rst)

  • if you don't have an issue number, change it to the pull request
    number after creating the PR

    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
    • .feature: A new behavior, public APIs. That sort of stuff.
    • .deprecation: A declaration of future API removals and breaking
      changes in behavior.
    • .breaking: When something public is removed in a breaking way.
      Could be deprecated in an earlier release.
    • .doc: Notable updates to the documentation structure or build
      process.
    • .packaging: Notes for downstreams about unobvious side effects
      and tooling. Changes in the test invocation considerations and
      runtime assumptions.
    • .contrib: Stuff that affects the contributor experience. e.g.
      Running tests, building the docs, setting up the development
      environment.
    • .misc: Changes that are hard to assign to any of the above
      categories.

  • Make sure to use full sentences with correct case and punctuation,
    for example:

    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

    Use the past tense or the present tense a non-imperative mood,
    referring to what's changed compared to the last released version
    of this project.

[codspeed-hq]

Merging this PR will not alter performance

✅ 59 untouched benchmarks

---

_{Comparing patchback/backports/3.13/4ec897bddfa84c23389d6bfe6764e549ba50b696/pr-12133 (f510bf0) with 3.13 (0ae2aa0)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.28%. Comparing base (0ae2aa0) to head (f510bf0).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.13   #12134   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files         128      128           
  Lines       44685    44685           
  Branches     2430     2430           
=======================================
+ Hits        43918    43919    +1     
  Misses        590      590           
+ Partials      177      176    -1     

Flag	Coverage Δ
CI-GHA	98.16% <ø> (+<0.01%)	⬆️
OS-Linux	97.91% <ø> (-0.01%)	⬇️
OS-Windows	95.61% <ø> (ø)
OS-macOS	97.18% <ø> (+0.01%)	⬆️
Py-3.10.11	96.71% <ø> (ø)
Py-3.10.19	97.19% <ø> (ø)
Py-3.11.14	97.38% <ø> (ø)
Py-3.11.9	96.90% <ø> (ø)
Py-3.12.10	96.99% <ø> (ø)
Py-3.12.12	97.47% <ø> (+<0.01%)	⬆️
Py-3.13.11	97.47% <ø> (-0.01%)	⬇️
Py-3.13.12	96.97% <ø> (-0.01%)	⬇️
Py-3.14.2	97.38% <ø> (-0.01%)	⬇️
Py-3.14.3	96.89% <ø> (ø)
Py-3.14.3t	96.72% <ø> (-0.01%)	⬇️
Py-3.9.13	96.60% <ø> (ø)
Py-3.9.25	97.08% <ø> (-0.01%)	⬇️
Py-pypy3.11.13-7.3.20	96.79% <ø> (+<0.01%)	⬆️
VM-macos	97.18% <ø> (+0.01%)	⬆️
VM-ubuntu	97.91% <ø> (-0.01%)	⬇️
VM-windows	95.61% <ø> (ø)

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