docs: Convert slack links to CNCF, add link to contributing guide

[vishwa-uber]

What changed?

Converts links from our old slack instance to the CNCF community inviter.
Adds a link to our shared contributing guide to the contributing readme.

Why?

The goal of the contribution guide is to move all common guides and processes between our repositories to a shared place – reducing conflicting information and streamlining the process to contribute to cadence.

How did you test it?

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.08%. Comparing base (d06d552) to head (d95868d).
⚠️ Report is 1 commits behind head on master.
see 2 files with indirect coverage changes

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