DOC: Improve user-facing documentation

[pllim]

As requested by @mustaric .

TODO

• Re-organize sections.
• Fix formatting: Heading level uneven, narrative + API mix hard to read.
• Improve docstring.

[mustaric]

This looks like a really good start. I don't have strong opinions on the organization, so if you like it, then I think we can go for it. If you have places where you are unsure we could discuss it.

[codecov]

Codecov Report

Merging #49 (f31330d) into master (cfe722b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
=======================================
  Coverage   80.75%   80.75%           
=======================================
  Files          18       18           
  Lines        1195     1195           
=======================================
  Hits          965      965           
  Misses        230      230           

Impacted Files	Coverage Δ
exovetter/model.py	90.32% <ø> (ø)
exovetter/vetters.py	86.13% <100.00%> (ø)

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[pllim]

Currently, the rendered API doc for vetters.py is wrong due to #54

[mustaric]

The rendered version of this looks good. There are places where I really should write better documentation about what each of these vetters do. I didn't see anything wrong, it is just very terse. I'd say go ahead and merge when you are ready.

[pllim]

Okay, I merged it. Thanks for the review! I'll continue in a separate PR after #54 is resolved.