Add docs for renovate configuration

[nathan-joslin]

There is an existing issue in renovate to add support for updating dagger module dependencies, with a "medium" priority.

In the short term, it may be worthwhile to add documentation to describe how to manually configure renovate to update our modules. In a comment in the linked issue, an example for bumping the version of the dagger engine itself is provided. We likely could modify this for our modules.

Follow-up:

• Consider how this may be done with dependabot or other tools similar to renovate, and if adding docs for them are worth our time.