hexdocs: improve visibility of currently supported functions

[diogonr]

This is a minor quality-of-life improvement.

Looking at https://hexdocs.pm/gleam_stdlib/gleam/function.html, the first 10 items on this list are deprecated functions, while only the last 3 are the actual properly supported functions, which are presumably what users want.

My suggestions would be to either:

1. Move the deprecated functions to the end of the list, or
2. Create a new section called "Deprecated" for these functions.