Consolidate readme.md and index.md

[kailuowang]

fix #1896
Right the two documents are highly duplicated. Using a sbt-microsite setting, we can use the same document both as the github readme and the index of the site.
After the consolidation, the readme file was a bit too long, so I extract the motivations section to a separate page.

[codecov-io]

Codecov Report

Merging #1915 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1915   +/-   ##
=======================================
  Coverage   95.54%   95.54%           
=======================================
  Files         248      248           
  Lines        4420     4420           
  Branches      121      124    +3     
=======================================
  Hits         4223     4223           
  Misses        197      197

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[ceedubs]

Nice. Thanks @kailuowang!

[kailuowang]

since this is a documentation only and straightforward change I am going to merge with one maintainer sign-off and one contributor sign-off.