docs: add DEPENDENCIES sections to function doc headers

Documents what each function needs for full functionality -- other
repo functions it calls, and external CLI tools it uses or falls
back gracefully without (e.g. rm/trash, ls/eza+lsd) -- matching the
existing CLASSIFICATION convention's placement and the ~20 functions
that already carried this label.

Also broadens verify-manual.py's dependency-resolution check to
recognize this repo's other existence-check idioms (command -q,
command -v, which -- not just type -q) and dng2avif's dynamic
type -q $cmd loop, since several genuine dependencies (eza, lsd,
fastfetch, fd, ps, ...) are only ever guarded that way.

CONTRIBUTING.md

@@ -390,7 +390,7 @@ all optional except where noted:
 |---|---|
 | `CATEGORY` | **Required to appear in the manual at all** — see below. |
 | `COMPONENT` | Only for functions gated by the [opinionated-component system](#opinionated-components). |
-| `DEPENDENCIES` | Other functions this one calls that a reader may want to look up. |
+| `DEPENDENCIES` | Other functions, and external CLI tools, this one needs for full functionality — required or optional-with-fallback alike. |
 | `CLASSIFICATION` | Hazard/shadow-interaction tags — see below. |
 | `SYNOPSIS` | One-line usage form. |
 | `DESCRIPTION` | Prose description; can span multiple paragraphs. |