Docs: Ignore build folder in doc8 (#676)

When documentation has been built and quality check is run afterwards,
it always fails because of errors in the docs/_build/ folder. This is
because doc8 is also scanning the generated documentation files for
errors.

This commit is adding an ignore-parameter to the call to doc8. With
this the build folder is ignored.

Author: jensvog

tox.ini

@@ -53,4 +53,4 @@ skip_install = true
 deps =
     sphinx
     doc8
-commands = doc8 docs/
+commands = doc8 --ignore-path docs/_build/ docs/