chore: Add snapshot update workflow

[filzrev]

This PR intended to resolve #9831 issue.

Currently snapshots are not synced to latest snapshot tests results.
This PR add snapshot,yml workflow to update snapshots.
And run ci.yml if snapshots are updated.

How to run snapshot update workflow

First of all. This PR must be merged to main branch to run workflow.

After PR is merged.
It can run workflow via GitHub Action Web UI.

It can also run workflow by using gh command.

gh workflow run snapshot.yml --repo dotnet/docfx --ref test-branch -f dryrun=true

How to test snapshot workflow

1. Ensure this PR is merged to main branch.
2. Create test branch (e.g. test-branch) and push to GitHub.
3. Run snapshot workflow with dryrun=false setting
4. Confirm snapshot is updated by workflow. And cy.yml is executed.

If it's confirmed to works as expected.
Please try to update main branch snapshot.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.92%. Comparing base (fe673ec) to head (40f095a).
Report is 183 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9969      +/-   ##
==========================================
+ Coverage   74.31%   78.92%   +4.60%     
==========================================
  Files         536      540       +4     
  Lines       23189    23433     +244     
  Branches     4056     4059       +3     
==========================================
+ Hits        17234    18494    +1260     
+ Misses       4853     3802    -1051     
- Partials     1102     1137      +35     

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[yufeih]

Thank you @filzrev ! This can make updating snapshots easier.