Easily accessible documentation of `x-zally-ignore`

[roxspring]

A colleague of mine recently asked whether x-zally-ignore declarations were cumulative as you descend the tree vs overriding. For example, given the yaml below should a TopRule violation pointing at /paths/~1things/get/summary be...

• ignored, because the cumulative ignore list is [TopRule, PathRule]
• allowed, because the overriding deeper ignore list is just [PathRule]

I hope it's the former, but wouldn't be surprised if it's the latter, and haven't got around to confirming.

swagger: 2.0
x-zally-ignore: [TopRule]
paths:
  /things:
    x-zally-ignore: [PathRule]
    get:
      summary: Gets all the things

All of which makes me think we should:

• [x] Decide intended behaviour
• [x] Potentially fix the implementation to match desired behaviour
• [ ] Document x-zally-ignore usage so that people understand
  • How to ignore at top level
  • Where to pull rule identifiers from
  • Where else x-zally-ignore can be used within a spec
• [ ] Include or reference this documentation from the Zally UI so that people have a fighting chance of discovering the functionality!

[roxspring]

Note that the example uses pre #754 syntax. Ideally, we'd not work on the documentation until that issue is closed, or else some documentation corrections will be needed as part of #754 too.

We shouldn't block on #754 to decide (and potentially fix) intended behaviour though.

[roxspring]

Note that the behaviour was addressed by making x-zally-ignore entries inheritable in #1073.

Clear and easily accessible documentation is still outstanding.

[vadeg]

Related to #1200