Improve lecture page to modernise and enable linking to community projects

[mmcky]

Summary

Improve the lecture page (https://quantecon.org/lectures/) to modernise it and enable linking to community projects like QuantMFR.

Motivation

The current lecture page could benefit from a fresh, modern design and the ability to showcase and link to community-driven projects that extend the QuantEcon ecosystem.

Proposed Changes

1. Modernise the design - Update the visual appearance of the lecture page to align with current web design standards
2. Enable community project linking - Add functionality to link to community projects such as QuantMFR and other related educational resources
3. Improve discoverability - Make it easier for users to find and navigate to related community projects from the lectures page

[jstac]

Thanks @mmcky , good calls!

We could use AI to produce a slightly better and more comprehensive abstract for each lecture. Maybe about the length of a paper abstract.