Add frictionless documentation

[TColl]

Fixes #492

I wrote the original code, so I guess it makes sense for me to have a stab at this one!

I've expanded the docstrings within pandera.io and then included a minimal new page within the documentation to render these more clearly - this feels like a more sensible option than writing documentation from scratch, given that we're only partially covering the frictionless feature set and this might change in future, so any changes will just need to be reflected in the accompanying docstrings to get pulled through to the documentation in future builds

[codecov]

Codecov Report

Merging #579 (697fce2) into master (36e0ea3) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
- Coverage   98.55%   98.52%   -0.03%     
==========================================
  Files          26       26              
  Lines        3255     3258       +3     
==========================================
+ Hits         3208     3210       +2     
- Misses         47       48       +1     

Impacted Files	Coverage Δ
pandera/io.py	100.00% <ø> (ø)
pandera/typing.py	97.05% <0.00%> (-1.41%)	⬇️

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[cosmicBboy]

thanks @TColl! Will handle the failing tests