Fix documentation building since recent changes in GPv3

The #109197 introduced the following error to the documentation
builders:

  Error: context key 'grease_pencil' not found in context_type_map; update ../doc/python_api/sphinx_doc_gen.py

To reproduce the issue locally the following command could be
used:

```
./bin/Blender.app/Contents/MacOS/Blender \
    --background \
    --factory-startup \
    -noaudio \
    --python ../../blender/doc/python_api/sphinx_doc_gen.py -- \
    --output ./sphinx/build_doc_api \
    --api-changelog-generate \
    --api-dump-index-path \
    ./sphinx/api_dump_index.json
```

Note that even after this fix the command from above would still
fail locally because the script expect information from previous
Blender version to be in the index. It is possible to bypass by
manually editing api_dump_index.json and adding 3.6 key.

Pull Request: https://projects.blender.org/blender/blender/pulls/109365

doc/python_api/sphinx_doc_gen.py

@@ -1185,6 +1185,7 @@ context_type_map = {
     "fluid": ("FluidSimulationModifier", False),
     "gpencil": ("GreasePencil", False),
     "gpencil_data": ("GreasePencil", False),
+    "grease_pencil": ("GreasePencilv3", False),
     "gpencil_data_owner": ("ID", False),
     "curves": ("Hair Curves", False),
     "id": ("ID", False),