MANIFEST.in: Only list the files from the docs folder that will be included (closes #2623)

Pruning the _build folder produced the message `no previously-included directories found matching 'docs/_build'` when installing from the source distribution.

MANIFEST.in

@@ -3,5 +3,4 @@ include test/*.py
 include test/*.json
 include youtube-dl.bash-completion
 include youtube-dl.1
-recursive-include docs *
-prune docs/_build
+recursive-include docs Makefile conf.py *.rst