Ensure documentation examples are linted and formatted

[StevenMaude]

Depends on #1342, as it's simpler to do this for standalone files than code inside Markdown.

[StevenMaude]

It doesn't really depend on #1342 really as there are ways of doing this already for inline code blocks. pytest-examples is one package that runs Ruff and Black against Markdown code blocks.

Ruff is maybe adding native support for formatting Markdown code blocks too.

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Whether we'd actually want to do this is another question. Particularly formatting with Black which can lead to extra parentheses as it splits long lines.

One option might be to define our own Ruff and Black configuration — maybe only Ruff as that now formats too — for formatting dataset definitions, which might not exactly be the same as we use for the ehrQL source.