Merge godocs into existing techdocs

If a project already has an existing `mkdocs.yml` with techdocs, godocs should go into a sub-nav item, perhaps named "Go Packages" by default.

Start by reading the existing mkdocs.yml handling and the godocs-to-TechDocs generation path. Verify how an existing techdocs navigation is represented, then confirm that Go Packages is added as a sub-navigation item without replacing the existing navigation; a project without existing techdocs should continue to work.

Written by the indexing model from the issue text.