Add Document Links example server

[alcarney]

Description (e.g. "Related to ...", etc.)

Continuing the migration towards end-to-end tests, this adds an example Document Links server where you can add support for custom link syntaxes

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

Automated linters

You can run the lints that are run on CI locally with:

poetry install --all-extras --with dev
poetry run poe lint

[alcarney]

Thanks! :)