Docs are missing clear pointers to `inproc` and similar

When translating Bash scripts, one of the most common tasks is to run a command and receive its output (or check its exit status). Turtle provides helpers for these (like `inproc` and `procs`), but the docs don't show them prominently so (despite looking at the docs several times before) I had no clue they existed until @Gabriella439 told me so directly!

Start by locating the project documentation for running commands and review how `inproc` and `procs` are currently referenced. Add prominent pointers explaining that these helpers return command output or expose exit status, then verify the documentation clearly directs Bash-script translators to them.

Written by the indexing model from the issue text.