docs: Add advice on presubmit & branches
docs: Add advice on presubmit & branches

Tested: By viewing the generated docs in a browser.
Fixed: b/223242211
Change-Id: Ia749294e5e9a950814260055ac1a51765a4694ba
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/89900
Pigweed-Auto-Submit: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>
Reviewed-by: Chad Norvell <f2b1b54074a2cea3f685a7cf4fc055999c2f0cd6@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed