API Docs: Fix xref urls

Fully revert D7913 "Fix T77276: Generating Python API docs raises many warnings"

{D7913} broke xrefs/links to other types and permalinks in the docs. This makes the python api docs for 2.90 and 2.91 completely unusable.
It got partially reverted in commit e893430a6306ded059270e7df9d78180cdc0d9e3. That didn't fix those two issues though, so it should be fully reverted.

As you can see here, i'm not able to click `bpy_struct.id_data`, because no <a> tag got generated in the html.
{F8889934}

Here you can see a working, but wrong permalink, generated by clicking the little chain. (It should be `bpy.types.MeshVertices`, not `bpy.types.MeshVertices.MeshVertices`)
{F8889938}

Reviewed By: Blendify

Differential Revision: https://developer.blender.org/D8913