Rewrite ruby buildpacks docs
Rewrite ruby buildpacks docs

- Explicitly highlight that database.yml is overwritten
- Explain that DATABASE_URL is automatically populated from VCAP_SERVICES
- Remove unnecessary example code.

[#79360702]

bridgetkromhout/docs-buildpacks,dantwining/docs-buildpacks,cloudfoundry/docs-buildpacks,cwest/docs-buildpacks,afeld/docs-buildpacks,TimGerlach/docs-buildpacks