Add 'Available since' to module API function docs (#10229)

The script which generates the markdown docs from module.c is updated to include
the version in which each module API function was introduced.

The script uses git tags to find this information. If git is not available or if we're not in
a git repo, the 'since' is silently skipped.

The line `**Available since:** (version)` is added after the function prototype

Rename to utils/generate-module-api-doc.rb