chore(ci): cleanup unused kokoro configs

[vishwarajanand]

Cleanup of unused kokoro configs

Changes made

1. Removed the .kokoro/docs/**, .kokoro/docker/** and .kokoro/publish-docs.sh
2. Removed the .kokoro/release.sh, .kokoro/requirements.in and .kokoro/requirements.txt since they are no longer a part of synthtool/gcp/templates/python_library/

Similar changes will be done in following repos once these are reviewed:

1. https://github.com/googleapis/langchain-google-alloydb-pg-python -> chore(ci): cleanup unused kokoro configs #429
2. https://github.com/googleapis/langchain-google-cloud-sql-pg-python/ -> chore(ci): cleanup unused kokoro configs langchain-google-cloud-sql-pg-python#307
3. https://github.com/googleapis/llama-index-alloydb-pg-python -> chore(ci): cleanup unused kokoro configs llama-index-alloydb-pg-python#126
4. https://github.com/googleapis/llama-index-cloud-sql-pg-python -> chore(ci): cleanup unused kokoro configs llama-index-cloud-sql-pg-python#127
5. Other Langchain** and llama** repos under googleapis org, if required.

Ref: b/402829221

[averikitsch]

This is custom to our pipeline. Do we need to set this up internally like our custom release.sh?

[vishwarajanand]

Thanks for pointing this out. I observed that the docs script is indeed customized for handling the package version.
So, I copied it internally: cl/768991918. The CL needs to be merged before this PR.