[SEO] add page description

[HerringtonDarkholme]

Google does not know what playground is doing

### Tasks
- [x] homepage
- [x] playground
- [x] introduction
- [x] quickstart
- [ ] pattern syntax
- [ ] project scan
- [ ] rule configuration
- [ ] atomic rule
- [ ] relational rule
- [ ] composite rule
- [ ] utility rule
- [ ] test rule
- [ ] tooling overview
- [ ] napi

[HerringtonDarkholme]

The internal AST query interfaces those tools offer are often poorly documented and difficult to write, understand, and maintain.