docs: fix contributing links and branch guidance

[HappyG1lmore]

Description

Fix stale and placeholder links in the contribution docs, and align branch guidance with the repository's default branch.

Changes Made

• Replace the TODO documentation link with the official Canopy docs URL
• Replace the placeholder Discord link with the active invite already used elsewhere in the docs
• Update README and CONTRIBUTING branch guidance from development to main

Testing

• Documentation-only change
• Verified the repository default branch is main
• Verified the development branch is not present via GitHub API