docs(code-coverage): add workspace-level README

[harivattappilli-hmda]

Hey, I just made a Pull Request!

Adds a workspace-level README.md for the code-coverage workspace, as requested in #4056.

Previously this workspace only had the default autogenerated README, which made it unclear that documentation actually lived in the individual plugin folders. The new README:

• Gives a brief description of what the workspace does (ingesting and displaying code coverage on entity pages)
• Lists the plugins in the workspace (code-coverage, code-coverage-backend) with links to their individual READMEs
• Summarizes installation steps for both the backend and frontend plugins
• Documents the backstage.io/code-coverage entity annotation (including the scm-only option)

Closes (partially) #4056 — checking off code-coverage in the workspace list there.

✔️ Checklist

• Added or updated documentation
• A changeset describing the change and affected packages. (more info)
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Thanks for the contribution!
All commits need to be DCO signed before they are reviewed. Please refer to the the DCO section in CONTRIBUTING.md or the DCO status for more info.

[04kash]

Hey @harivattappilli-hmda, looks like you've modified the wrong file (the README in the 3scale workspace instead of the README in the code-coverage workspace). You'll want to make your changes to https://github.com/backstage/community-plugins/blob/main/workspaces/code-coverage/README.md instead.

Also make sure that your commit is signed (https://github.com/backstage/community-plugins/pull/10295/checks?check_run_id=93860716961)