Alter Sphinx makefiles to use docs/build directory

The documentation utility Sphinx creates makefiles for compiling this
project's documentation media. Prior to this commit, compiled feature
code and compiled documentation code were placed together in the
build/ directory. This commit alters Makefile and make.bat to output
documentation media to docs/build to keep documentation and compiled
feature code separate.
Merge pull request #3 from osu-cass/maint-3802_SphinxDocs

Alter Sphinx makefiles to use docs/build directory