[tests] Add rstcheck and sphinx checks

- pywikibot-core-tox-rstcheck-docker is for rstcheck run
- pywikibot-core-tox-sphinx-docker for sphinx check with small
  themes like "basic" or "nature" which runs within 5 minutes
- pywikibot-core-tox-doc-docker is for "furo" sphinx theme later
  and for pywikibot-core-tox-publish CI run
- use multiple processes to build distribution

Bug: T322979
Change-Id: Ifbcd9716d68dfd6206fd75aebff5163d4bd2d9b3
---
M tox.ini
1 file changed, 32 insertions(+), 0 deletions(-)

diff --git a/tox.ini b/tox.ini
index dbed3d0..69ebd75 100644
--- a/tox.ini
+++ b/tox.ini
@@ -88,12 +88,27 @@
 basepython = python3.7
 commands =
     sphinx-build -M html ./docs ./docs/_build -j auto
+deps =
+    -rrequirements.txt
+    -rdocs/requirements.txt
+
+[testenv:rstcheck]
+basepython = python3.7
+commands =
     rstcheck --version
     rstcheck -r --report-level WARNING --ignore-roles api,phab .
 deps =
     -rrequirements.txt
     -rdocs/requirements.txt
 
+[testenv:sphinx]
+basepython = python3.7
+commands =
+    sphinx-build -M html ./docs ./docs/_build -j auto -D html_theme=nature
+deps =
+    -rrequirements.txt
+    -rdocs/requirements.txt
+
 [flake8]
 # The following are intentionally ignored, possibly pending consensus
 # D105: Missing docstring in magic method

