add a quick start guide and rearrange buildspec documentation

[shahzebsiddiqui]

move content of buildspec into 'Writing Buildspecs' section Add a 'Getting Started' caption on left hand side

[codecov]

Codecov Report

Base: 71.41% // Head: 71.41% // No change to project coverage 👍

Coverage data is based on head (da2ba25) compared to base (5c09915).
Patch has no changes to coverable lines.

❗ Current head da2ba25 differs from pull request most recent head c8c1c2e. Consider uploading reports for the commit c8c1c2e to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #1335   +/-   ##
=======================================
  Coverage   71.41%   71.41%           
=======================================
  Files          57       57           
  Lines        6093     6093           
  Branches     1081     1081           
=======================================
  Hits         4351     4351           
  Misses       1740     1740           
  Partials        2        2           

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