This adds support for a new attribute `_example` which, if present, will
load the file under `tests/examplefiles` pointed to by `_example` and
show it as part of the lexer documentation. This allows us to have
unit-tested files show up as small code snippets in the documentation,
without having to worry that the documentation output contains possibly
incorrectly highlighted code.

Also tweak the style a bit so code snippets in admonitions look
acceptable.

Anteru deleted the anteru/lexer-examples branch

Anteru added the A-docs

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