DOC: Show module docstring

[lgolston]

Rationale

A remaining issue with the latest docs, comparing against https://scitools.org.uk/cartopy/docs/v0.17/, is that the module level docstring is not currently showing but can contain very useful information.

Implications

This is fixed simply using automodule instead of module. However, in some cases documentation has been added to the rst file directly, which I have either transferred to the corresponding module/class, or combined if there was already a docstring, which should be better for maintainability of the information. I have have also tried to reduce repetition of 'this module... ' in several places for readability purposes.

[lgolston]

Thanks for approving @dopplershift. Just wanted to make two small changes, which are added now:

• Combined text in one more place in cartopy.io I had missed.
• Added a link in Feature to FeatureArtist with regard to where zorder of 1.5 is defined, since that doc was slightly confusing.