Add ability to generate ReleaseNotes document from fragments

[Claudenw]

Our current release process requires that we update the release notes by editing a the RELEASE_NOTES.md in the docs/src/site/markdown directory`.

This proposal is to allow the creation of markdown files in the form version.txt in the docs/srs/site/markdown directory.

process

1. The scripts/release_detail.sh script is run and the output redirected to docs/src/site/markdown/version.txt.
2. The version.txt file is edited for clarity and completeness.
3. The create release pr action is executed
4. The resulting PR is verified and merged
5. The create release action is execute.
6. A new release is created at: https://github.com/Aiven-Open/bigquery-connector-for-apache-kafka/releases/
7. The contents of the version.txt file is copied into the release notes.
8. the release is finalized.

advantages

• The contents of the version.txt file is generated from git and then cleaned up.
• Once cleaned the version.txt file is used consistently.
• Multiple branches can be handled at the same time.
• As branches are added the website places the release notes in the proper order.

additional code

• A velocity plugin in tools to generate the final output.
• Separate version.txt files created from the current RELEASE_NOTES.md

example code.

Example of the velocity plugin and text files is found at
https://github.com/Claudenw/bigquery-connector-for-apache-kafka/tree/build_release_notes