Various doc enhancement

Here are some doc. enhancement, including:

* fix path to `scripts/loader.py`, previously in `python/examples`,
* remove of all Sphinx related markup, allowing perfect rendering through GitHub,
* use explicit `~/PhiloLogic4` root path to all paths behind repository,
* add a "downloading" section in `INSTALLING.rst`,
* mention `pip` as an option for installing Python packages,
* add a new `requirement.txt` document,
* change how database loading was presented, now starting by describing what will result of loading be,
* add mention of common options to `loader.py`,
* fix some old typos.