Add doc links to primitives.featurelabs.com

[thehomebrewnerd]

Add links to primitives.featurelabs.com

Closes Issue #858

Added links in
-README.md
-Feature Primitives documentation page
-Primitives FAQ page
-Featuretools Enterprise page
-API Reference
-Sidebar

[kmax12]

Codecov Report

Merging #860 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #860   +/-   ##
=======================================
  Coverage   98.14%   98.14%           
=======================================
  Files         117      117           
  Lines       10812    10812           
=======================================
  Hits        10611    10611           
  Misses        201      201           

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[kmax12]

Nice.

I think let's also mention it here: https://docs.featuretools.com/en/stable/api_reference.html#feature-primitives

And can we add a direct link to it under "RESOURCES AND REFERENCES" in the side bar? Right below API reference? Call it "Primitive Reference".

[kmax12]

LGTM. in case you're curious, I built the docs online to preview the change while I was reviewing: https://docs.featuretools.com/en/issue858/