integrating discord links to readme and about page

[runningonsideprojects]

Fixes #3613

Changes:

• [p5.js-web-editor readme]: Adding the discord invitation link to the Community Section
• [editor.p5.org/about]: Updating the discord invitation link on the About Page:
  • "Join the Discord" hyperlink under Get Involved section
  • "Discord" hyperlink of the Socials row of the Contact section

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks so much for the updates on these! :)