docs: add guide for adding tested doc examples

[Leokaufi]

Task

Closes #254

Description

Adds a short guide to docs/development.md explaining how to add a new
documentation page with a tested code example: where to put the snippet
file (docs/examples/<category>/), how to include it in the .md page via
pymdownx.snippets, how the snippet is picked up by the test
(tests/test_docs/test_doc_examples.py, run as a script via runpy), and how
to build/preview the docs with mkdocs.

Guide lives in docs/development.md for now. The open question from the issue
(docs page vs. README vs. CONTRIBUTING) is not hard-decided — happy to move it
if reviewers prefer another location.

Note on the two illustration blocks: the snippet-marker and include
examples carry a leading ; (e.g. ;--8<--). This is the pymdownx.snippets
escape so the markers render literally on the built docs site instead of being
interpreted and expanded at build time. The ; is stripped in the rendered
HTML but is visible when reading the raw .md on GitHub — so it's intentional,
not a typo.

How Has This Been Tested?

Docs-only change, no code. Ran uv run --dev mkdocs build — builds without
errors, and both illustration blocks render literally as the intended syntax
(verified in the generated HTML). prek hooks pass on the commit.

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.

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