MAINT: use a function to test the warnings of the CI doc build

[12rambau]

FIX #937

new stuff:

• A file listing all the warnings that we tolerate when the documentation is build. This list only show the warning after the keyword (WARNING or ERROR) to avoid issue with directory name (nox, not nox, hosted, virtualenv) and the file seperator between Linux and Windows
• A test script that works in 3 steps:
  1. verify that all the expected warnings are here
  2. list the unexpected one
  3. if one of these list is not empty return 1 which stop the Action
• some documentation in contribution on how to add new WARNING to the list

Let me know what you think

[choldgraf]

This looks nice, thanks - just a few quick comments. I'll also push a commit to clean up the language a bit, hope that's OK

[choldgraf]

I feel like this is going to get tedious because every new image here will create toil where we have to update these warnings. Could we allow this to support glob-like patterns and then do _static/gallery/*?

[12rambau]

the advantage here is that in 1 loop as I pop the element from the list I get both missing and unexpected. We already have a lot of examples in the gallery do you really expect it to increase that much?

[choldgraf]

maybe not - let's not block this PR on that question, we can revisit if it becomes cumbersome or if people forget.

[choldgraf]

A couple quick thoughts, this is looking pretty good to me though!

[choldgraf]

maybe not - let's not block this PR on that question, we can revisit if it becomes cumbersome or if people forget.

[choldgraf]

LGTM - thanks for this improvement to the maintainability of the theme!