move doc build checks from travis to readthedocs PR checks

[graingert]

Currently builds are checked with tox/travis, but the readthedocs environment is different/more restrictive than travis.

It would be better to have readthedocs build and check the docs for PRs rather than travis: https://docs.readthedocs.io/en/stable/guides/autobuild-docs-for-pull-requests.html

It's probably worth leaving the docs test in tox for local development

[hippo91]

@graingert i think it is a good idea. I would like to know what @Pierre-Sassoulas , @AWhetter and @PCManticore think about it.

[Pierre-Sassoulas]

Sounds good to me too.

[graingert]

steps that are needed:

go here: ​https://readthedocs.org/dashboard/pylint/advanced/#id_external_builds_enabled and tick this box ​

then screenshot the "Default settings" fromset, it should look something like this: I can use that to create a .readthedocs.yml file

then go here ​https://readthedocs.org/dashboard/pytlint/integrations/create/ to double check the webhook is enabled

[graingert]

@Pierre-Sassoulas or I can set this up, with admin access to the pylint repo and readthedocs project

[Pierre-Sassoulas]

I'm a pylint-admin now, let's do this :D