Fix typos and update path of 'scripts/loader.py'.
Remove all Sphinx related markups.
Uniformise all `variables` and `path` to be marked as ``literals``.
Use ``~/PhiloLogic4`` as path of local clone to PhiloLogic4 repository.
Add a "downloading" subsection to installing doc.
Add a requirement document.
doc: Phrasing, add link and emphasing.
doc: Mention use of `pip` to install `philologic` Python module.
doc: Restructure and better explain loading.
doc: Rephrasing, merging notes on `pip` usage and ``requirement.rst`` document, and other misc.
doc: Remove from ``requirement.rst`` document softwares needed for 3rd-party installations.
Merge pull request #135 from vincent-ferotin/doc-enhance

Various doc enhancement