Content: add a page for a gallery, to show off some examples of documenation

[agjohnson]

This is a common request, and something that we've accumulated in a few various places. It would be great to show off what user generated documentation can look like. Here's a good example:

https://quarto.org/docs/gallery/

[pradyunsg]

Such pages are typically called showcases, and because I’m actually researching this stuff myself, here’s a bunch of links:

• https://jekyllrb.com/showcase/
• https://gatsbyjs.com/showcase/
• https://gohugo.io/showcase/
• https://docusaurus.io/showcase/
• https://getdoks.org/showcase/

Gallery is typically used to show a bunch of features of a tool/library/project visually.