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:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

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