Improve documentation for adaptdl ray-aws

[rmfan]

• Fix dead links
• Change tutorial steps
• Add a sample cluster configuration
• Add a new section describing a bit of how it works

[codecov-commenter]

Codecov Report

Merging #127 (7a7c539) into master (3453440) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #127   +/-   ##
=======================================
  Coverage   61.38%   61.38%           
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  Files          32       32           
  Lines        2530     2530           
  Branches      416      416           
=======================================
  Hits         1553     1553           
  Misses        896      896           
  Partials       81       81           

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