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

[rem1niscence]

the development branch information is incorrect, merges should be done against the main branch at the moment, the actual code intended to be used in validators are the versioned releases but we do expect PR's to be merged againstmain

[HappyG1lmore]

Thanks for clarifying. The current PR diff updates the branch guidance to target main only. I left the versioned-release validator note out of CONTRIBUTING to keep this patch focused, but can add that nuance if you want it documented there too.