[source-build-release] Improve checklist clarity in various process steps

[NikolaMilosavljevic]

Checklist clarity could be improved, by being more explicit, and having a little bit more information. We should strive to avoid relying on 'tribal knowledge'.

Examples of missing info:

• installer commit is referenced a dozen of times in the checklist, but it is never described how this commit can be discovered. Solution: describe how to find installer build from stage-dotnet - via manifest
• dry-run is mentioned as required step. It is not clear what if anything should be validated in dry-run outputs (outputs of various steps, artifacts, etc) Solution: be very specific about validation.
• If pipeline fails and a fix is needed, user needs to investigate the pipeline sources (ymls). Solution: specify that sources are available in public source-build repo
• It is important that release Tag points at the correct commit. Various pipeline inputs and source changes could produce unexpected results and cause tagging to point at incorrect commit. Solution: Add a new steps in 3.d. for tag verification.
• There is some confusion around trigger for Approval - Release stage, the SYNC POINT. Current instructions, "Wait for Microsoft build release", are not clear enough. Solution: Add something along these lines: for 8.0 wait for public release blog announcement, for 6.0/7.0, besides waiting on public release blog announcement, also verify that release tags are available in public repos.

[NikolaMilosavljevic]

Fixed with https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/34494