Update and expand users guide

[MJWeberg]

Fairly large update to the User's Guide.

Summary of changes:

• Expanded intro section about EIS observations to include a quick data primer and moved it all to a new chapter.
• Gave more details on how to use eis_catalog and eis_browse_templates
• Updated cropping and other details changed by NDcube 2.0
• Added more MPFIT documentation (and reformatted its doc string)
• Added links to the API documentation
• Updated a few function and class doc strings in EISPAC
• Set readthedocs yaml to now generate a PDF copy of the docs (I hope)