docs: fix contributing links

[HappyG1lmore]

Description

Fix two broken links in CONTRIBUTING.md:

• replace the TODO documentation link with the Canopy docs URL used in the repository README
• replace the placeholder Discord invite with the active invite already referenced earlier in the same file

Related Issues

N/A

Changes Made

• Updated the documentation link in the issue guidance section
• Updated the Discord link in the larger-change coordination section

Checklist

• I have tested the changes locally and verified they work as intended.
• I have appropriately titled my branch.
• I have run re-built the web-wallet and/or block explorer (if applicable).
• I have run npm run prettier to format the web-wallet and/or block explorer (if applicable)
• I have updated documentation (if applicable).
• I have included tests for the changes (if applicable).

Additional Notes

Documentation-only change; no code, build, or generated assets changed.