docs-issue-generation: rewrite the script in go

[jlinder]

The build/teamcity-docs-issue-generation.sh script needs to be extended in ways beyond what we should do in a bash script. Rewrite the script in go so it can be extended to do what it needs to do.

A suggested approach:

• Create the new application in the pkg/cmd/docs-issue-generation directory
• Translate the existing script's logic into go.
• Update the existing script to build and run the new command
• Backport the changes to the release-21.2 branch

Epic: DEVINF-261

Jira issue: CRDB-13554

[github-actions]

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[rickystewart]

This is done thanks to @nickvigilante .