Documentation developer guide and errors

[ElliottKasoar]

The current developer docs do not describe stages of the setup required to build the docs, such as setting up add@localcode.

It's also unclear if/why installing docs/requirements.txt is suggested, on top of .[docs].

I'm not sure if these are setup issues or actual issues, but I get various warnings/errors when building the docs, such as:

• TypeError: timeout argument must be a number or None, got Float from wg.run() in ../gallery/howto/autogen/monitor.py
• AttributeError: 'NoneType' object has no attribute 'get_content' from wg.outputs.stdout.value.get_content()) in ../gallery/howto/autogen/shelljob.py
• aiida-workgraph/docs/source/howto/autogen/interoperate_with_aiida_core.rst:148: ERROR: Unexpected indentation.
• aiida-workgraph/docs/source/howto/autogen/interoperate_with_aiida_core.rst:151: WARNING: unknown document: '../../gallery/howto/autogen/annotate_inputs_outputs'
• Many other blankline/underline warnings