Improve Demo Implementation and Links for kits

[lindakatcodes]

With our new design live, the "Demo Implementation" section of the kit README files is redundant. We also have some extra projects that have used specific starter kits that we'd like to add to the new showcases component.

This ticket should handle removing that section from each README that has it, and add relevant showcases that might be missing to the SHOWCASES array. (Some of how showcases are stored now might change based on #1292, so please keep an eye on the status of that ticket if it gets resolved before this one gets worked on.)

Acceptance criteria:

• [ ] Each kit's README file should have the "Demo Implementation" section removed if it contains it