Update gixy documentation reference to actively maintained fork

[dvershinin]

The original yandex/gixy repository has been unmaintained since 2020.

The dvershinin/gixy fork (published as gixy-ng on PyPI) is actively maintained with:

• Python 3.6+ support (up to 3.13)
• Additional security checks beyond the original
• Regular updates and releases
• Modern dependency support

The documentation structure is the same, so the link still works.

Official website: https://gixy.org
Documentation: https://gixy.getpagespeed.com

[MegaManSec]

Heads up: there are multiple forks of gixy. I maintain gixy-next here: https://github.com/megamansec/gixy-next / https://gixy.io/
If you are choosing which fork to link to, please consider some blog posts I've written: https://joshua.hu/gixy-ng-new-version-gixy-updated-checks#quality-degradation and https://joshua.hu/gixy-ng-ai-slop-gixy-next-maintained.

Thank you.

[dvershinin]

@MegaManSec Your gixy-next fork has demonstrably lower code quality - I've reviewed it and found issues that would not pass basic code review. Desperately copy-pasting the same comment across all my PRs because your own submissions weren't merged is not a constructive approach. Trash-talking AI tooling in blog posts won't get your fork mentioned anywhere credible.

For maintainers: gixy-ng is actively maintained since 2020, has over 30 security checks (more than the original), supports Python 3.6-3.13, is published on PyPI with proper versioning, and has a proper documentation site at https://gixy.getpagespeed.com. The claims of "advertising" refer to a single informational line about the maintained fork - standard practice for any active fork.