Share doctree between builders

We are reducing build time with this.
And also saving a little of space in the builders.

According to sphinx, we are safe doing this:

> the doctrees can be shared between all builders

http://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-d

Closes #4363

Author: stsewd

readthedocs/doc_builder/backends/sphinx.py

@@ -217,7 +217,7 @@ def build(self):
             '-b',
             self.sphinx_builder,
             '-d',
-            '_build/doctrees-{format}'.format(format=self.sphinx_builder),
+            '_build/doctrees',
             '-D',
             'language={lang}'.format(lang=project.language),
             '.',