Determine ideal dependabot update frequency

[legoktm]

Currently (c.f. #6) we have dependabot update dependencies on a weekly basis (note that this does not include security updates, as those happen "immediately"). I think we should continue to try this out for a month or so and then re-evaluate whether weekly is the right cadence or whether something slower (e.g. monthly) would be better.

Also we may want different frequencies for docs repos vs the different SD component repos.