add additional data sources to vector DB

[tatianab]

This is an umbrella issue to track work on adding additional data sources to the vector DB which feeds the related entities API (#22). Each data source should get its own issue as we decide which to include.

Currently, we fully support GitHub issues as a data source. Other data types possibly worthy of inclusion are:

• CLs (enable Gerrit sync #20)
• Documentation (add data source: go.dev documentation #26)
• Code documentation
• Code
• Issue comments / threads