Blender Git Loki

Addons: Allow a user manual url prefix in doc_url

This was raised in T74017, the issue being that we point to `/dev`
version of the manual for the addons when we want to point to a specific
version instead.

Instead of manually updating the URL every release we can do this.

The `bl_info` for addons will need to be updated in the format of
`'doc_url':
{BLENDER_MANUAL_URL}/addons/import_export/scene_gltf2.html",`

Differential Revision: https://developer.blender.org/D6995