Update docs for PRNGKeyArray union and minimum Python version

[rishabhsai]

docs/api/array.md documents jaxtyping.PRNGKeyArray as a two-member union, but #388 added UInt32[jax.Array, "4"] for the rbg/unsafe_rbg PRNG implementations and the page has not been touched since before that merged, so a reader would conclude an old-style four-uint32 key is rejected; import jaxtyping; print(jaxtyping.PRNGKeyArray) prints typing.Union[jaxtyping.Key[Array, ''], jaxtyping.UInt32[Array, '2'], jaxtyping.UInt32[Array, '4']].

Separately, README.md and docs/index.md both say "Requires Python 3.10+." while pyproject.toml has requires-python = ">=3.11", raised in #380, so pip install jaxtyping refuses to install on 3.10 while the README promises it works.

This updates the union line, extends its parenthetical to say which implementations give four uint32s, changes both "3.10+" lines to "3.11+", and uv run mkdocs build still succeeds.