Add documentation for vscode remote development with sfdx

[dehru]

What does this PR do?

this PR adds documentation about the sfdx support for VS Code Remote Development. This PR is cherry-picked from 1603 which is already merged into develop ( #1603 )

What issues does this PR fix or reference?

W-6212362

[codecov]

Codecov Report

Merging #1609 into release/v46.12.0 will decrease coverage by 0.01%.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##           release/v46.12.0   #1609      +/-   ##
===================================================
- Coverage             71.81%   71.8%   -0.02%     
===================================================
  Files                   183     183              
  Lines                  7337    7337              
  Branches                843     843              
===================================================
- Hits                   5269    5268       -1     
  Misses                 1894    1894              
- Partials                174     175       +1

Impacted Files	Coverage Δ
...forcedx-vscode-core/src/channels/channelService.ts	83.33% <0%> (-2.39%)	⬇️

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