Readd Transf

[james-d-mitchell]

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[james-d-mitchell]

I think this is ready to go, @Joseph-Edwards

[Joseph-Edwards]

Modulo the tests passing (which I assume they will after libsemigroups/libsemigroups#544 is merged) and my minor suggestion above, this looks good to me. That said, I'd like to build and read the docs just to be doubly sure. I'll do this first thing tomorrow morning.

[Joseph-Edwards]

I've read had a more thorough read through the docs now, and it all looks very good to me. I have a few minor comments before I'd be happy to merge.

1. For consistency, should the new pages in the doc have a table of contents as in Action and Knuth-Bendix?
2. After the signatures of some of the functions, some return types have hyperlinks and some don't. For example, in PPerm(dom: List[int], img: List[int], deg: int) → StaticPPerm16 | PPerm1 | PPerm2 | PPerm4, StaticPPerm16 gets a hyperlink, but the others don't. It's not immediately clear if there's a nice fix for this (or if a nice fix is even necessary), but it would be nice to do if there is.
3. If I understood correctly, @james-d-mitchell asked if it was possible to show the constructor when documenting a class. I couldn't remember how to do that at the time, but I now recall that is done with something like this:

.. autoclass:: StaticTransf16
   :class-doc-from: class
   :members:
   :inherited-members:
   :special-members: __init__

With the above comments considered, I would be happy to merge