📖docs: Add .sigstore bundles to Signed-Releases

[cpswan]

What kind of change does this PR introduce?

docs

• PR title follows the guidelines defined in our pull request documentation

What is the current behavior?

.sigstore bundles were added to Signed-Releases check by #3772 but docs weren't updated

What is the new behavior (if this is a feature change)?**

Docs will reflect checks as they presently are in main

NA - [ ] Tests for the changes have been added (for bug fixes/features)

Which issue(s) this PR fixes

Fixes #3914

Special notes for your reviewer

checks.yaml was edited and then checks.md was generated with make generate-docs, but as it's a trivial change there's no obvious evidence of the docs generation process (which is probably as things should be).

Does this PR introduce a user-facing change?

Yes:

Document that `.sigstore` bundles are part of check for Signed-Releases

[codecov]

Codecov Report

Merging #3922 (610e2e5) into main (d55dbd1) will decrease coverage by 4.58%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3922      +/-   ##
==========================================
- Coverage   75.08%   70.51%   -4.58%     
==========================================
  Files         234      234              
  Lines       15857    15857              
==========================================
- Hits        11907    11181     -726     
- Misses       3189     3965     +776     
+ Partials      761      711      -50