Change test_extrusion_lsw python demo to rst demo

[ZiyaSelim]

Description

Fixes #3864

[connorjward]

Thanks!

Please also add this to the docs here. Before merging we'll also need to check that this renders correctly on the website. You can do this in the Docker container by going to firedrake_repo/docs and running make html.

[connorjward]

Unfortunately this is not passing tests:

1	FAILED firedrake-repo/tests/firedrake/demos/test_demos_run.py::test_serial_demo[extruded_shallow_water/extruded_shallow_water] -   File "/tmp/pytest-of-root/pytest-25/popen-gw0/test_serial_demo_extruded_shal0/extruded_shallow_water.py", line 21
1	    power = 5
1	IndentationError: unexpected indent

Looking at the RST file there appears to be inconsistent indentation for the Python snippets.

[connorjward]

Awesome work. Thanks so much! I will merge if tests pass.

[connorjward]

All done. Thank you!