docs(quay): enhance workspace-level README

[AhnfLabib]

What does this PR do?

Replaces the default scaffolded README in workspaces/quay with a workspace-specific overview. The new README describes the Quay workspace purpose, lists its four packages (quay, quay-backend, quay-actions, quay-common) with links and short descriptions, and adds a quick-start install snippet that points to the per-plugin READMEs.

Why was this PR needed?

Issue #4056 asks for stronger workspace-level READMEs so contributors can understand what each workspace contains. Investigation showed that workspaces/analytics (my original target) was already improved in #6852, while workspaces/quay still used the boilerplate "This is your newly scaffolded Backstage App, Good Luck!" text despite being a multi-plugin workspace. This change follows the format used by gold-standard workspace READMEs (e.g. announcements) and the precedent of #6852.

This is a follow-up to closed PR #9538. Per maintainer feedback:

• Removed the workspace README regression test (docs-only PRs do not need that coverage)
• Signed the commit with DCO (Signed-off-by: Ahnaf Labib <ahnaflabib212@gmail.com>)

What are the relevant issue numbers?

Addresses #4056

Screenshots / Recordings (if applicable)

Documentation-only change (no UI or backend runtime). Before/after of workspaces/quay/README.md:

Before (scaffold):

# Backstage

This is your newly scaffolded Backstage App, Good Luck!

To start the app, run:

yarn install
yarn start

After: workspace title + purpose, Plugins section linking all four Quay packages, and Quick start install commands.

• Rendered after: https://github.com/AhnfLabib/community-plugins/blob/fix-issue-4056/workspaces/quay/README.md

Does this PR meet the acceptance criteria?

• Tests added for new/changed behavior — N/A (docs-only; no runtime behavior; test removed per maintainer request on Fix issue 4056 #9538)
• All tests passing — CI checks should run on this PR (DCO, quay workspace CI/verify)
• Follows project style guide — matches existing enhanced workspace README pattern (docs: improve workspace-level READMEs for selected workspaces #6852 / announcements)
• No breaking changes introduced
• Documentation updated (if applicable)
• All commits have a Signed-off-by line

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[christoph-jerolimov]

This looks good to me now. Thanks for this improvement.