Summary:
This updates the grist-core README to list specific features of Grist,
to make it easier for a casual visitor to get a sense of its scope. Adds links
to some new resources (reviews, templates, grist v airtable post) that could
also help. Adds python3 to docker image so that templates work without fuss.

Test Plan: existing tests should pass

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal, anaisconce

Differential Revision: https://phab.getgrist.com/D3204