docs: Add keyword docstring for `typegroup` (#61907)

`typegroup` had a section in the manual but no keyword docstring, so
`?typegroup` at the REPL returned no documentation. Add a
`kw"typegroup"` entry alongside `struct` and `mutable struct` in
`base/docs/basedocs.jl`.

This pull request was written with the assistance of generative AI
(Claude).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>