Update download link and refactor the documentation directory

[FeynmanZhou]

Fix https://github.com/notaryproject/notation/issues/307

[FeynmanZhou]

cc @notaryproject/notation-code-reviewers

[codecov-commenter]

Codecov Report

Merging #308 (3d0ab25) into main (e73bfc3) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #308   +/-   ##
=======================================
  Coverage   30.66%   30.66%           
=======================================
  Files          25       25           
  Lines        1611     1611           
=======================================
  Hits          494      494           
  Misses       1104     1104           
  Partials       13       13           

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