Show short endorsements from well-known people at the top of the README #221

Part of #183. Split from #88, which refreshed the README's first screen (#220).

Problem

A newcomer decides in a few seconds whether to stay. Kalman and Bayesian Filters in Python and d2l put short quotes from well-known people near the top of their README, so a visitor sees right away that people they know recommend the project. This project has been recommended publicly (for example in posts that shared it), but the README doesn't show it.

Expected change

• Collect one to three short quotes from people who have recommended the project publicly
• Ask each person for permission to quote them with their name and role (the maintainer does this)
• Show the quotes under the summary and the links on the README's first screen, above the demo grid
• Keep the full list in USERS_COMMENTS.md

Done when

The README shows at least one quote with the person's permission.