Add asset implementation description in DG

[yisiox]

Updated the DG with some details of the design of asset related features.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.89%. Comparing base (40636d3) to head (e131ec1).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #157   +/-   ##
=========================================
  Coverage     78.89%   78.89%           
  Complexity      488      488           
=========================================
  Files            72       72           
  Lines          1464     1464           
  Branches        119      119           
=========================================
  Hits           1155     1155           
  Misses          285      285           
  Partials         24       24           

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[bryanyee33]

LGTM