manual: add "Emacs" section to manual

This gives some basics on configuring Emacs within Nix. The
configuration is fairly long just to give a good idea of what’s going
on. I can trim out some of it if it’s not necessary. Note that there
is already a section for Emacs in the NixOS manual. However, this is
aimed at avoiding using modules altogether to make things easier for
non-NixOS users. This configuration should work on NixOS anyway,
however.

Fixes #24243
Fixes #19956