Document new jetbrains-backend-coder repository

[Kira-Pilot]

We should add documentation and CI to our new repository.

Requirements:

• Add a CI workflow for automatic release and publishing.
• Support (maybe via separate branches) both early-access and LTS versions as the JetBrains API is not stable and may have breaking changes.
• Add a comprehensive README that details the usage of the plugin as well as the CI workflow and versioning required above.
• Ask JetBrains if we can push to the JetBrains marketplace (since it is using internal APIs)

[Kira-Pilot]

We've added a README; that's all the documentation for now as we cannot publish - closing out.