Document conventions for reusable workflows

## Is your feature request related to a problem? Please describe.
We've started some conventions with our reusable workflows, e.g. `workflow_reusable.yml`, though it'd be nice to explicitly note this and tidy up how they appear on PRs.

## Describe the solution you'd like
A note in contributing.md on what we're doing here with reusable workflows generally and then details of any naming conventions to follow.

A standard practice for naming that makes use of the 3 tier job hierarchy we've created

E.g. instead of the lintr one here

We could do something like `dfeshiny / reusable / lintr` (or the reverse, or something similar)

## Describe alternatives you've considered
None, I think we should document somewhere and make more use of the job naming though.

## Additional context
None.

Start with contributing.md and the existing workflow_reusable.yml workflow, then inspect the lintr workflow naming shown in the issue. Document the current reusable-workflow conventions and specify the agreed three-tier job naming pattern; done means the guidance is recorded and the relevant workflow job names follow it.

Written by the indexing model from the issue text.