Install alibi-detect during RTD docs build

[ascillitoe]

With the ReadTheDocs V2 config file it appears we can install alibi-detect (and its dependencies) as part of the docs build, so that they don't have to be mocked. This is a bit of a waste of compute whilst we have heavy core deps like tensorflow, but once the optional deps project is complete we can investigate installing alibi-detect on RTD and only mocking heavy optional deps (e.g. tensorflow, torch). This would ease pain points experienced when building the API docs involving some mocked modules such as pydantic.

Opening this PR now to come back to once the optional deps project is complete.

If going ahead with this we'd likely have to configure :inherited-members: to prevent docstrings of inherited modules being included when we don't want them to e.g. see example here.

[ascillitoe]

This change might also allow us to https://autodoc-pydantic.readthedocs.io/en/stable/index.html to prettify our pydantic model api docs. This package doesn't appear to work properly if pydantic, or any modules used as types in model fields, are mocked... (although this would depend what is mocked...)