[tox] Add docs-iterative command

This is useful for iterative work with docs as it avoids trashing
results from previous builds and thus saves time.

Change-Id: Ic4f710725cbc2d85c28ca4e00c3a32a00a72c2da

tox.ini

@@ -50,6 +50,13 @@ deps =
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build/html
+  {[testenv:docs-iterative]commands}
+
+[testenv:docs-iterative]
+envdir = {toxworkdir}/docs
+skip_install = {[testenv:docs]skip_install}
+deps = {[testenv:docs]deps}
+commands =
   sphinx-build -n -W --keep-going -b html doc/source doc/build/html
   whereto doc/source/_extra/.htaccess doc/test/redirect-tests.txt