DOCS: Use Sphinx directives to include code _using tags_

[JDBetteridge]

Describe the issue
Documentation currently uses line number references to include code snippets from the test suite (see, for example: https://github.com/firedrakeproject/firedrake/pull/3309/files). This is incredibly fragile to any edits made to the test suite. We should instead use

start-after/start-at/end-before/end-at

Sphinx directives, as documented here

Where to find

• All over, but a good starting place would be docs/source/interpolation.rst

Additional info
Discussed in meeting 2024-01-10.