SC2: update resources listing

[trvrb]

Description of proposed changes

This commit makes a few small changes to the SARS-CoV-2 resources listing:

• Adds a link to /sars-cov-2/forecasts
• Updates abstract text
• Removes link to /groups/blab/sars-like-cov (less relevant three years in)
• Removes link to discussion.nextstrain.org (links to discussion forum are now much more prominent in docs pages)

Testing

Tested locally and via Heroku review App.

• [x] Checks pass