Show migration advice for apps using `bin/{pre,post}_compile` scripts

[edmorley]

Apps using the proprietary bin/{pre,post}_compile scripts should migrate to using the upstream CNB inline buildpack feature: https://buildpacks.io/docs/for-app-developers/how-to/build-inputs/use-inline-buildpacks/

We should:

1. Make the build error if an app has one of those scripts, instructing them to remove the script and migrate to the inline buildpack. This error should give an example of what to add to project.toml, and link to the upstream docs.
2. Make some improvements to the upstream docs (eg: show the shorter syntax style, and also give an example using the io.buildpacks.post.group feature)

See:

• https://github.com/heroku/buildpacks-python/issues/14
• https://buildpacks.io/docs/for-app-developers/how-to/build-inputs/use-inline-buildpacks/

GUS-W-17140857.