Update links to learning and community resources

[bgins]

This PR makes the following changes:

• [x] Remove note and link to UIPs repo. We briefly had a repository for improvement proposals, but it no longer exists.
• [x] Direct learners to GH working group where they can find multiple implementations
• [x] Remove link to the Fission Whitepaper
• [x] Link to the UCAN Discord server instead of the Fission Discord server
• [x] Add a link to the UCAN community call