Add documentation about how to inject other dependencies into pipx devtools

[offbyone]

pipx supports "injecting" other dependencies, as libraries or binaries, into pipx installs: https://pipx.pypa.io/latest/docs/#pipx-inject

This allows the composition of tools, like say yt-dlp and gallery-dl into a single environment; the injection persists across updates.

It'd be nice to have that documented in here somehow, assuming it's even possible with mise.