Read The Docs documentation

[tkknight]

Iris documentation overhaul to use a modern responsive theme and to be served via https://readthedocs.org/. This is the new baseline for future improvements to be made.

• Added page for contributing to the documentation
• Adopted sphinx_rtd_theme (responsive to device, standard formatting)
• Adopted sphinx-copybutton
• Adopted sphinx-gallery and removed the custom one. Updated references to the new gallery
• Added "make html-noplot" option to skip the gallery creation (fast when developing)
• Renamed the examples_code and example_tests to be gallery_code and gallery_tests
• Prefixed all gallery files with "plot_"
• Ensured a clean build with no warnings (there were many)
• Setup to now build by readthedocs (see the config file in the root .readthedocs.yml that uses conda)
• Added travis test to include "make linkcheck" that checks that all urls are valid - fixed many that were not
• Updated image on iris github homepage to be better resolution and standard font (Georgia)
• Build directory now in src directory, named "_build"
• Remove legacy files from "_static"
• Sphinx conf.py file has better structure and comments
• Copyright date range end is now dynamic
• Various typos and formatting in source
• API rst builder now show no warnings when run
• Added link to the "legacy" documentation
• Put in place a block list for troublesome modules when generating the API documentation
• Changed "Whitepapers" to "Technical Papers"

[bjlittle]

@tkknight Super awesome... thanks for all you effort on this.

Just some minor changes, then we're good to go! 👍

[tkknight]

I think this is good to go now.

[bjlittle]

@tkknight LGTM 👍

Just waiting for travis-ci to do it's thing...

As a follow-up, I can now tackle re-hauling the iris requirements, thanks!

[trexfeathers]

Resolves #3690