docs: init design doc for OCI Store Cache Worker

[junczhu]

Description

What this PR does / why we need it:

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):

Design for #1110

Todos:

• [ ] create memory backed content store for ORAS store in ratify
• [ ] create task queue in cache provider
• [ ] create cache worker, link cache worker with ORAS store, cache provider
• [ ] link OCI store with cache provider task queue (using cache worker interface)
• [ ] check cache data flow

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