Add "Branch Guidance" docs page for docs and samples repos

[willdavsmith]

What content needs to be created or modified?

The Git branch strategy for docs is different than most projects (i.e., we use release branches as the main branches and edge as prerelease. We should provide a "Branch Guidance" docs page to clear up any confusion about this.

Describe the solution you'd like

A page similar to https://docs.dapr.io/contributing/docs-contrib/contributing-docs/#branch-guidance which outlines how we handle branching for docs and samples.

AB#13042