docs: rescue broken sigstore docs link

[achrinza]

Changes

Updates link to point to new location after the docs refactor

see: sigstore/docs#210

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
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  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
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Release Notes

Updates link to point to new location after the Sigstore docs refactor

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: achrinza / name: Rifa Achrinza (6144f03)

[jkhelil]

@achrinza please add a release note

[achrinza]

done.

[jkhelil]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.95%. Comparing base (f7d1ba7) to head (6144f03).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1886   +/-   ##
=======================================
  Coverage   61.95%   61.95%           
=======================================
  Files          64       64           
  Lines        4071     4071           
=======================================
  Hits         2522     2522           
  Misses       1268     1268           
  Partials      281      281           

Flag	Coverage Δ
unit-tests	61.95% <ø> (ø)

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[anithapriyanatarajan]

/lgtm