Update documentation for lbexec

The only commit to review here is 55d61da9, the rest are covered by !722 (merged).

The most notable changes other than replacing davinci-specific documentation with lbexec-specific documentaton:

• I tried to include python code from the test files rather than hard coding it so it's more likely to work (even if the text might become outdated). In particular this fixes several outdated examples. As part of this I reformatted a couple of the examples to make them be more reasonably auto-formatted.
• I use autodoc_pydantic to automatically document the Options class.

All of this documentation could definitely be improved however I think this is a good enough start.

For reference - related to DPA tasks https://gitlab.cern.ch/lhcb-dpa/project/-/issues/102 and https://gitlab.cern.ch/lhcb-dpa/project/-/issues/180.