Extend pages for API documentation

[danielhuppmann]

Please confirm that this PR has done the following:

• Documentation Added
• Description in RELEASE_NOTES.md Added

Description of PR

This PR fell into a few rabbit holes (after starting off as a minor clean-up prior to release of v0.5).

• separate the API doc page into several subpages, add functions that weren't previously documented (general functions, variables utils, read_datapackage())
• update all references in the tutorial notebooks and other docs pages (because of changes in the item above)
• add numpydoc reference to "contributing" guidelines
• set up intersphinx for pandas and numpy
• harmonize formatting and use :code:-style less often to improve readability (make pages less busy)
• switch to more-numpydoc-compatible docstrings
• fix a bunch of "not-displayed-as-intended" issues

This branch is rendered on readthedocs.

[coveralls]

Coverage decreased (-0.05%) to 85.797% when pulling 50b23fc on docs/clean-up_v0.5 into ce03f5a on master.

[gidden]

Reviewed online and looks great! Thanks a bunch @danielhuppmann

[danielhuppmann]

merging myself because the PR only concerns documentation and built doc pages spot-checked by @gidden