Broken RTD builds

[davidorme]

Describe the bug

ReadTheDocs have changed their recommended recipe for installing from poetry - the current setup does not install the documentation requirements to the environment used by the RTD sphinx build, so it all fails (and has been for a month!).

To Reproduce

Look at ReadTheDocs build page: https://readthedocs.org/projects/pyrealm/builds/

We can fix this in .readthedocs.yaml using the new recommended set up:

readthedocs/readthedocs.org#11152

And as implemented here:

https://github.com/ImperialCollegeLondon/virtual_ecosystem/pull/398/files