Docs: update perfetto UI release Process
Docs: update perfetto UI release Process

Add docs for cherry-picks and add more rationale about the
current 4 weeks process.

Change-Id: Ifd913f7318a169b6f92b39c3cbc32211e87cbe53
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto