[Docs] Add markdown support

- Use recommonmark to enable use of markdown files in docs.
- Fix theme not specified
- Remove unused spelling module.
- Cleanup mocking modules in conf so building docs requires only Sphinx.
- Simplify tox section, including use of requirements-docs file. Added
  slimit dependency for sdist-ing deluge package.

tox.ini

@@ -164,10 +164,8 @@ commands =
 sitepackages = False
 changedir = docs
 deps =
-    {[testenv]deps}
-    sphinx==1.7.*
-whitelist_externals =
-    {[testenv]whitelist_externals}
+    slimit
+    -rrequirements-docs.txt
 
 [testenv:docs]
 sitepackages = {[docsbase]sitepackages}
@@ -182,10 +180,9 @@ sitepackages = {[docsbase]sitepackages}
 changedir = {[docsbase]changedir}
 deps =
     {[docsbase]deps}
-    coverage
     pytest-cov
 whitelist_externals =
-    {[docsbase]whitelist_externals}
+    pytest
     mkdir
 commands =
     mkdir -p build/doccoverage