doc/hooks/just: add documentation

Introduced in #247147

Part of #341479

doc/hooks/index.md

@@ -16,6 +16,7 @@ gdk-pixbuf.section.md
 ghc.section.md
 gnome.section.md
 installShellFiles.section.md
+just.section.md
 libiconv.section.md
 libxml2.section.md
 meson.section.md

doc/hooks/just.section.md

@@ -0,0 +1,23 @@
+# `just` {#just-hook}
+
+This setup hook attempts to use [the `just` command runner](https://just.systems/man/en/) to build, check, and install the package. The hook overrides `buildPhase`, `checkPhase`, and `installPhase` by default.
+
+[]{#just-hook-justFlags} The `justFlags` variable can be set to a list of strings to add additional flags passed to all invocations of `just`.
+
+## `buildPhase` {#just-hook-buildPhase}
+
+This phase attempts to invoke `just` with [the default recipe](https://just.systems/man/en/chapter_23.html).
+
+[]{#just-hook-dontUseJustBuild} This behavior can be disabled by setting `dontUseJustBuild` to `true`.
+
+## `checkPhase` {#just-hook-checkPhase}
+
+This phase attempts to invoke the `just test` recipe, if it is available. This can be overrided by setting `checkTarget` to a string.
+
+[]{#just-hook-dontUseJustCheck} This behavior can be disabled by setting `dontUseJustCheck` to `true`.
+
+## `installPhase` {#just-hook-installPhase}
+
+This phase attempts to invoke the `just install` recipe.
+
+[]{#just-hook-dontUseJustInstall} This behavior can be disabled by setting `dontUseJustInstall` to `true`.