Update documentation

[noahrhodes]

restructure documentation pages

• Home: basic how to make a plot
• Examples
  • Basic Plots: how to change data, color
  • Advanced Plots: advanced modifications using vegalite internals and set-field
• Parameters: explain all of the possible attribute arguments to use
• Experimental Features: how to try out experimental features

partially closes #39 closes #3 (through documentation) closes #48

[codecov-commenter]

Codecov Report

Merging #58 (a5654b8) into master (c2d74cb) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #58   +/-   ##
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  Coverage   92.04%   92.04%           
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  Files          13       13           
  Lines         352      352           
=======================================
  Hits          324      324           
  Misses         28       28           

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