docs: replace gitter links with discord

[Selflocking]

resolved: https://github.com/casbin/casbin-website-v2/issues/179

[casbin-bot]

@seriouszyx @ComradeProgrammer @Resulte please review

[codecov[bot]]

Codecov Report

Merging #45 (7ea1731) into master (432c626) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #45   +/-   ##
=======================================
  Coverage   20.65%   20.65%           
=======================================
  Files           2        2           
  Lines          92       92           
  Branches       20       22    +2     
=======================================
  Hits           19       19           
  Misses         67       67           
  Partials        6        6           

[github-actions[bot]]

:tada: This PR is included in version 0.11.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: