Define design for Renovate usage

[mthalman]

This design document describes my proposal for how Arcade can provide the use of Renovate by consuming repos. The use of Renovate is an important part of https://github.com/dotnet/dotnet-buildtools-prereqs-docker/issues/1321 which will greatly simplify the process of keeping consuming repos up-to-date with supported images. It be used in other scenarios besides Docker, such as needing to keep up-to-date with 3rd party products which provide releases through GitHub.

The language used in the doc describes things as if they already exist, so it'll continue to be correct and relevant once this is all implemented.

My ask is for the dnceng team to own and manage the bot account. I'm happy to implement everything else.

@mmitche - Please add any other reviewers you think should see this.

[MichaelSimons]

cc @markwilkie, @ilyas1974

[markwilkie]

I really do like the fact that the bot writes to a fork.