Documentation Cleanup & Improvements for 0.8.7 release

[cgewecke]

• Remove HARDHAT_README.md (vestigial - doesn't seem to be necessary in HH docs anymore)
• Add trouble shooting with advice to use hardhat clean and transpile only
• Add TOC to advanced topic, deprecate file system topics
• Add link to DRIPS

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (44a999f) 56.37% compared to head (20143bc) 96.36%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #859       +/-   ##
===========================================
+ Coverage   56.37%   96.36%   +39.98%     
===========================================
  Files          15       19        +4     
  Lines         816     1017      +201     
===========================================
+ Hits          460      980      +520     
+ Misses        356       37      -319     

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