New markdown link checker and linter are overly sensitive

[samodell]

Many PRs in the Docs repo right now have build test failures when it seems like they shouldn't.

Causes:

• Oversensitive link verification, such as serving/samples: remove EXPOSE directive from helloworld #644 (more details in the comments of that PR; but to summarize, one link is valid and causing a failure, and one link is to a http://localhost URL, which we should probably exclude from link checks)

• Unknown: I can't see a root cause for some failures, such as Add link to image in Dockerhub #747
  Error logs are vague:
  I0110 23:28:45.331] ============================
  I0110 23:28:45.332] ==== BUILD TESTS FAILED ====
  I0110 23:28:45.332] ============================
  W0110 23:28:45.363] Run: ('/workspace/./test-infra/jenkins/../scenarios/../hack/coalesce.py',)
  E0110 23:28:45.363] Command failed
  I0110 23:28:45.363] process 643 exited with code 1 after 0.7m
  E0110 23:28:45.363] FAIL: pull-knative-docs-build-tests

CC @srinivashegde86 @jessiezcc @adrcunha

[adrcunha]

About the links, yes, the checker might be over sensitive. Probably it doesn't like an address without protocol, and I'm pretty sure localhost:4000 is not returning content at the time of checking. :)

About the second, scroll up the logs and you'll see:

I0110 23:28:24.004] ------------------------------------
I0110 23:28:24.005] ---- Linting the markdown files ----
I0110 23:28:24.005] ------------------------------------
I0110 23:28:26.564] serving/samples/helloworld-go/README.md:141: MD007 Unordered list indentation
I0110 23:28:26.565] 
I0110 23:28:26.565] A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

[samodell]

@adrcunha with #747, I played around with the list formatting and wasn't able to satisfy the linter. The docs for the linter say that you must use 2 spaces for indented items in lists, but when I used that level of indentation, it broke the numbered list the unordered list is under by making the next numbered item in the list start over at 1.

I think this linter might not be a great fit for our repo. Is there a way to disable the Markdown linter but keep the link checking capabilities?

[adrcunha]

Yes, hunting markdown linting errors this way is counterproductive. I'll disable the linter for docs.

[adrcunha]

knative/test-infra#391 allows disabling the linter for a repo.