Update Discord channels references in unit 0

Hey! :)

We have noticed (@sergiopaniego ) that the references to some Discord channels in the file \chapters\en\unit0\welcome\welcome.mdx are outdated. Specifically, these channels do not exist anymore in the Discord HF server:

- Line 55: `#cv-study-group` -> has changed to `cv-community-project`
- Line 58: `#diff-i-made-this` -> has changed to `#i-made-this`

I'll open a PR requests to make these changes.

Thank you!