docs: add notaryv2 upgrade doc

[binbin-li]

Description

What this PR does / why we need it:

As we updated notaryv2 to notation across the project, we would need to add a section explaining how to help users update the config and constraint template.

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 #978

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 coverage has no change and project coverage change: -0.94% ⚠️

Comparison is base (b600277) 59.11% compared to head (fd0b2a5) 58.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #999      +/-   ##
==========================================
- Coverage   59.11%   58.17%   -0.94%     
==========================================
  Files          73       91      +18     
  Lines        4432     5447    +1015     
==========================================
+ Hits         2620     3169     +549     
- Misses       1567     1964     +397     
- Partials      245      314      +69     

see 37 files with indirect coverage changes

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[akashsinghal]

Slightly unrelated but on our readme, there's one notary v2 reference that was missed. Could we just add that to this PR?

[susanshi]

LGTM. thanks!