[docs] Document cu2qu library
Reorganise the documentation so that everything is in one place and users are more clearly pointed to the modules which are likely to be useful for their purposes. (I still think it’s worth having at least a brief reference to ``cu2qu.cli`` in there, as a way of reminding users that there is a command-line implementation.) Docstrings are provided for non-API methods where I could understand them - trusting these will be useful for future maintainers.
https://tox.readthedocs.io/en/latest/config.html#conf-skip_missing_interpreters
Runing `tox` with no options runs the tests agaist all the python
environments listed in the `tox.ini`'s `envlist` (currently 3.6, 3.7 and 3.8).
Before this change, if any of these versions was not available, tox would exit with an error. Now it will simply continue (with a warning).
This can be useful when on a developer box, one might only have a subset of all our supported interpreters installed but we don’t want to mark the build as failed because of it.
Note that on the CI I am passing the opposite command line switch to override this setting, because there I want to make sure none of the specified interpreters is skipped.
[docs] Document feaLib
* Rearrange docs by user intention, highlighting the things you can do with each component.
* Remove reference to lexer and error modules from documentation tree, since they’re not user-facing.
* I’ve added docstrings to the parser even though we only provide access to the user-facing part of the API in the main documentation, just to clarify what some of the more obscure methods do and provide links to the spec.
* AST *is* user-facing if you’re building your own feature files in code, so all classes are documented with the user in mind.
Does not add any new documentation (there is already some documentation for user-facing functions). It just makes colorLib.builder the top-level documentation entry, because that’s the part the user needs to care about.
* List all modules and utilities
* Move wordy optional requirements to separate page
* Remove cruft from README and add pointers to online documentation
