docs: move cosign doc to website

[akashsinghal]

Description

What this PR does / why we need it:

Please merge ONLY AFTER notaryproject/ratify-web#30 has merged

Moves cosign document to website. Adds placeholder redirect text.

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 #

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

All modified and coverable lines are covered by tests ✅

Comparison is base (a04be82) 52.39% compared to head (f959c39) 52.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1168   +/-   ##
=======================================
  Coverage   52.39%   52.39%           
=======================================
  Files         101      101           
  Lines        6344     6344           
=======================================
  Hits         3324     3324           
  Misses       2699     2699           
  Partials      321      321           

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