Update docs code snippets to be taken from test files

### What

In the documentation we have hand-written code snippets. These are not compiled, so may fall out of sync with the actual code.

This task is to add files for these code snippets that are built as part of the module build, and then include them directly from the src files in the documentation.

### Acceptance Criteria (DoD)

All code snippets in the user documentation is included from a src file that is built by bazel.

### How

Use sphinx literalinclude rather than hand-written code blocks.

Start by inventorying the hand-written snippets in the user documentation and inspecting the module's Bazel build and existing src files. Create built sources for the snippets and replace documentation code blocks with Sphinx literalinclude directives. Done means every user-documentation snippet is included from a src file built by Bazel.

Written by the indexing model from the issue text.