Add gdscript entry in new parser docs

[Davidy22]

Quick page written up for #3199.

[masatake]

Thank you.
I'll merge this after merging the parser.
I have a plan to write a man page. I would like you to review it in the future.

[codecov]

Codecov Report

Merging #3206 (48b36a7) into master (1d9e6fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3206   +/-   ##
=======================================
  Coverage   84.84%   84.84%           
=======================================
  Files         205      205           
  Lines       48369    48369           
=======================================
  Hits        41040    41040           
  Misses       7329     7329           

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