docs: rename doc/ to docs/
docs: update class and module names
docs: switch to armstrong Sphinx template
docs: change docs title to just python-pptx
docs: add dynamic version display

Version used to be hard-coded, requiring update before each release.
Now it pulls from the standard location, pptx.__version__.
docs: migrate installation to separate page
docs: mv _build dir to .build
docs: rough-out user guide structure
docs: rename developer/ to dev/
docs: rough-out Contributor Guide
docs: rough out Community Guide
docs: migrate release history to HISTORY.rst
docs: migrate vision to dev/philosophy.rst
docs: migrate LICENSE to proper spot
docs: clean out redundant README.rst bits
docs: change README.rst to included module
docs: migrate running tests under dev/
docs: reorganize API documentation
docs: clean out dev/ API pages
docs: add Useful Links sidebar