chore(sdk): Add links to dotnet and js sdk

[seeflood]

What this PR does:
delete gitmodules;
add README and links to dotnet and js sdk instead.

Which issue(s) this PR fixes:

I just found that git submodules can only be linked to a fixed commit id. It cannot link to the latest commit of a branch.
So I decide to use a README instead

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

Merging #317 (8ee3b2c) into main (382f08e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #317   +/-   ##
=======================================
  Coverage   56.90%   56.90%           
=======================================
  Files          48       48           
  Lines        2100     2100           
=======================================
  Hits         1195     1195           
  Misses        774      774           
  Partials      131      131           

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