Docs: file: list `read` and `write` again (#49838)
Point 5 of https://github.com/JuliaLang/julia/issues/43428
Motivation: I feel like the most fundamental *file* operations in Julia
are `read(filename::String)` and `write(filename::String, content)`, so
it makes sense to repeat their documentation in the **Filesystem**
documentation. I also see
these methods as fundamental to teach beginners.
Like https://github.com/JuliaLang/julia/pull/49837, the goal is to only
show short docstrings for these methods, not the whole `IOStream`
functionality. So it depends on
https://github.com/JuliaLang/julia/pull/49835 and
https://github.com/JuliaLang/julia/pull/49836 to be useful :)
