Add documentation for fixtures

[alcaeus]

Fixtures currently aren't documented at all. This can be difficult, especially since fixture services are relying on being tagged (which is done using the ODMFixtureInterface interface, but users need to know that.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed in a week if no further activity occurs. Thank you for your contributions.

[smilesrg]

Still? 😄