Add GitHub Repository listing

[oleg-nenashev]

We expect the number of hosted projects to grow, and it would be nice to have a projects listing in a card layout, with a support of grouping and filtering. For inspiration, it could be a layout like https://blueswen.github.io/portfolio/

FTR blueswen/blueswen.github.io#2 for conversing that sample code to a plugin

[oleg-nenashev]

Sample from oleg-nenashev/repo-listing.github.io#1