docs: add examples for using AWS Signer

[byronchien]

Description

What this PR does / why we need it:

Adds documentation on using Gatekeeper/Ratify to verify signatures with notation and AWS Signer.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #871

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0d03e79) 54.87% compared to head (2ab1e5a) 54.87%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #875   +/-   ##
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  Coverage   54.87%   54.87%           
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  Files          81       81           
  Lines        4798     4798           
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  Hits         2633     2633           
  Misses       1883     1883           
  Partials      282      282           

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[akashsinghal]

lgtm

[susanshi]

thanks for the PR @byronchien , the content looks good to me! wondering if AWS Signer is the currently recommended approach , and if we should highlight that. @jimmyraywv , let us know if you have any feedback before we merge this. thanks!

[byronchien]

thanks for the PR @byronchien , the content looks good to me! wondering if AWS Signer is the currently recommended approach , and if we should highlight that. @jimmyraywv , let us know if you have any feedback before we merge this. thanks!

yes, using AWS Signer with notation on AWS is the recommended approach.