Add support for generated documentation for content type packages

[muthu-mps]

Summary

The README generation during package build is supported for type integration and input. This helps generating the Alerting Rule Template docs automatically. Currently, the spec doesn't allow generated README for content type packages.

Make changes to the spec to support /_dev/build/docs/README.md file for content type package.

These changes are required for autogenration of the README, which will be required for rendering the alert and slo templates in the readme

[muthu-mps]

cc: @lalit-satapathy @ishleenk17