Fix Discord Links in contributing.md and index.md

[MartinKong1990]

What was wrong?

Broken Discord links, same issue with https://github.com/ethereum/fe/pull/849 but on different pages.

How was it fixed?

Old: https://discord.com/invite/yCT6NYBb

New: https://discord.com/invite/ywpkAXFjZH

To-Do

• [ ] OPTIONAL: Update Spec if applicable

• [ ] Add entry to the release notes (may forgo for trivial changes)

• [ ] Clean up commit history

[cburgdorf]

Thanks!