docs: refactor-docs-structure

[nisiyong]

What's the purpose of this PR

Make the document structure more clear. The summary is as follows:

• Add 4 sections: SDK Guide, Admin Guide(Portal), Extension Guide(Portal Extension), Contributor Guide.
• Remove 2 sections: Development, and Usage.
• Each language SDK has one single document.

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (73b4b84) 49.59% compared to head (aea6f48) 49.65%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5084      +/-   ##
============================================
+ Coverage     49.59%   49.65%   +0.06%     
- Complexity     1907     1908       +1     
============================================
  Files           372      372              
  Lines         11564    11564              
  Branches       1127     1127              
============================================
+ Hits           5735     5742       +7     
+ Misses         5486     5479       -7     
  Partials        343      343              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[nobodyiam]

LGTM