Add nutshell documentation for bouncerscript behavior.

[MihaiTabara]

Until we figure out what's the best place for this docs to live, let's have it here for now.

[coveralls]

Pull Request Test Coverage Report for Build 138

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 132:	0.0%
Covered Lines:	316
Relevant Lines:	316

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💛 - Coveralls

[MihaiTabara]

@escapewindow It you think it's worth it, I'm happy to move this to readthedocs in scriptworker and centalize documentation around there. I have some nutshell beetmoverscript docs as well that I don't know what to do with.

[escapewindow]

I think this is the right place, as opposed to scriptworker. It might also make sense to expand on the in-tree docs... https://firefox-source-docs.mozilla.org/taskcluster/taskcluster/kinds.html?highlight=bouncer#release-bouncer-sub is a bit light. We also have in-depth, in-tree release promotion docs here; bouncer may deserve a page.

The kinds doc source is here. The in-depth relpro docs are linked from here and point at other individual pages in that directory, e.g. balrog.

Now that we have these docs in the bouncerscript repo, I'm fine with leaving the in-tree docs for a later followup. (Or, if you want to tackle them now that you have momentum and context, I'm good with that too.) Thank you!

[MihaiTabara]

@escapewindow I've merged this to make it live in the main repo. I'll take some time to copy-paste this in-tree. Sounds like bouncer definitely deserves a page. Thanks for all the pointers.