docs: document append_layout and sway-save-tree

Add sway(5) entry next to assign and a sway-save-tree(1) page;
register the new manpage in meson.build.

swaysavetree/sway-save-tree.1.scd

@@ -0,0 +1,52 @@
+sway-save-tree(1)
+
+# NAME
+
+sway-save-tree - Dump a workspace's tiling tree as append_layout-compatible JSON.
+
+# SYNOPSIS
+
+_sway-save-tree_ --workspace <name>
+
+# DESCRIPTION
+
+Reads the live tree of the running sway instance over the IPC socket, walks
+the named workspace, prunes runtime fields, and emits a JSON document
+suitable for *swaymsg append_layout*(5). Each leaf view becomes a
+placeholder entry whose *swallows* array contains a regex-anchored match on
+the view's *app_id* (Wayland) or *class* / *instance* (xwayland).
+
+The output is sent to standard output. Pre-existing placeholders that were
+loaded by *append_layout* and have not yet swallowed a window are echoed
+verbatim from their *swallows* array.
+
+This is the sway counterpart of i3-save-tree(1).
+
+# OPTIONS
+
+*--workspace* <name>
+	Name (or number, as a string) of the workspace to dump. Required.
+
+*-h, --help*
+	Show help and exit.
+
+# LIMITATIONS
+
+Floating windows are skipped in this release; only tiling children of the
+workspace are emitted. A warning is printed to stderr when floating windows
+are present.
+
+# EXAMPLES
+
+Save the layout of workspace 1 to a file, kill the running windows, and
+restore the layout:
+
+```
+$ sway-save-tree --workspace 1 > /tmp/layout.json
+$ swaymsg [workspace=1] kill
+$ swaymsg append_layout /tmp/layout.json
+```
+
+# SEE ALSO
+
+*sway*(5), *sway*(1), *swaymsg*(1)