[DOCS] Update Discord links to the newest one

[Panquesito7]

What would you like to share?

The current Discord invitation link is the default Discord link that looks unclean.
It should be updated to the latest one we're using, which is https://the-algorithms.com/discord.

Steps to contribute

1. Fork the repository here.
2. Create a new branch named update_invitation_link.
3. Open the README.md file.
4. Replace all discord.gg/ instances with https://the-algorithms.com/discord, which is the new link we're using for invitations to our community.
5. Commit and push your changes.
6. Create a pull requst to compare your changes.
7. You're done now! 🎉 You just have to wait until the maintainers review and check your PR.

If you need any help with this, let us know! 🙂

Additional information

This is a good first issue. Please leave it open to first-time contributors and beginners.

[tianyizheng02]

I guess this isn't really relevant anymore now that I merged your PR #8696...