Generics details part 1 (#553)

This proposal goes into the details of the core of the generics feature, to achieve the goals from #24 , and provides an outline covering future work. It has been summarized in these presentations:

- [basic usage](https://docs.google.com/presentation/d/1OZiMTVW2Ommop5WTs9RyEwnGxy9yzaAPF7Cj5KUfDsY/edit?resourcekey=0-Nya0Soz3ZNs3hJan8VIrTA#slide=id.p)
- [details: interfaces](https://docs.google.com/presentation/d/1FSlqtE5hXZIwOO52UrAK9DINBLDWtgu24dugHfomUMg/edit#slide=id.p)
- [details: facet types](https://docs.google.com/presentation/d/17KG0TeJ4OChMRdLJPS8TE_K6SoL4lFy1FUGr2CDzX-A/edit?resourcekey=0-kLnZqd5NrbGSwmbunTyB-A#slide=id.p)
- [details: type-types](https://docs.google.com/presentation/d/1Hn3VDlVjwhjx3SKM2KXKE7lW208nXff30x3-uIO4_Fo/edit#slide=id.p)
- [details: extending/refining interfaces](https://docs.google.com/presentation/d/1K0cCHeb9JTJY9QCGEVO9CcJNHYlaXkoPESv4J9tl5LU/edit#slide=id.p)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

docs/design/generics/README.md

@@ -16,5 +16,6 @@ feature of Carbon:
     direction.
 -   [Terminology](terminology.md) - A glossary establishing common terminology
     for describing the design.
--   ~~Detailed design~~ - not implemented yet
+-   [Detailed design](details.md) - In depth description of how generic type
+    parameters work.
 -   ~~Rejected alternatives~~ - not implemented yet