move linkcheck out of main doc build

Running linkcheck from github actions causes too many issues with rate
limiting or bot-protected sites. Disable it.

Author: dhellmann

tox.ini

@@ -38,6 +38,11 @@ deps =
   -r{toxinidir}/docs/requirements.txt
 commands =
     sphinx-build -W -j auto -b html -d docs/build/doctrees docs/source docs/build/html
+
+[testenv:linkcheck]
+deps =
+  -r{toxinidir}/docs/requirements.txt
+commands =
     sphinx-build -W -j auto -b linkcheck -d docs/build/doctrees docs/source docs/build/linkcheck
 
 [testenv:pkglint]