docs: Move the spawn SECTION

Move it to a separate page.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/glib/glib.toml.in

@@ -68,6 +68,7 @@ content_files = [
   "testing.md",
   "atomic.md",
   "threads.md",
+  "spawn.md",
   "random.md",
   "markup.md",
   "base64.md",

docs/reference/glib/meson.build

@@ -171,6 +171,7 @@ expand_content_files = [
   'host-utils.md',
   'data-structures.md',
   'shell.md',
+  'spawn.md',
   'string-utils.md',
   'unicode.md',
   'uuid.md',

docs/reference/glib/spawn.md

@@ -0,0 +1,76 @@
+Title: Spawning Processes
+SPDX-License-Identifier: LGPL-2.1-or-later
+SPDX-FileCopyrightText: 2014 Red Hat, Inc.
+SPDX-FileCopyrightText: 2017 Philip Withnall
+
+# Spawning Processes
+
+GLib supports spawning of processes with an API that is more
+convenient than the bare UNIX [`fork()`](man:fork(2)) and
+[`exec()`](man:exec(3)).
+
+The `g_spawn_…()` family of functions has synchronous ([func@GLib.spawn_sync])
+and asynchronous variants ([func@GLib.spawn_async],
+[func@GLib.spawn_async_with_pipes]), as well as convenience variants that take a
+complete shell-like command line ([func@GLib.spawn_command_line_sync],
+[func@GLib.spawn_command_line_async]).
+
+See [class@Gio.Subprocess] in GIO for a higher-level API that provides
+stream interfaces for communication with child processes.
+
+An example of using [func@GLib.spawn_async_with_pipes]:
+```c
+const gchar * const argv[] = { "my-favourite-program", "--args", NULL };
+gint child_stdout, child_stderr;
+GPid child_pid;
+g_autoptr(GError) error = NULL;
+
+// Spawn child process.
+g_spawn_async_with_pipes (NULL, argv, NULL, G_SPAWN_DO_NOT_REAP_CHILD, NULL,
+                          NULL, &child_pid, NULL, &child_stdout,
+                          &child_stderr, &error);
+if (error != NULL)
+  {
+    g_error ("Spawning child failed: %s", error->message);
+    return;
+  }
+
+// Add a child watch function which will be called when the child process
+// exits.
+g_child_watch_add (child_pid, child_watch_cb, NULL);
+
+// You could watch for output on @child_stdout and @child_stderr using
+// #GUnixInputStream or #GIOChannel here.
+
+static void
+child_watch_cb (GPid     pid,
+                gint     status,
+                gpointer user_data)
+{
+  g_message ("Child %" G_PID_FORMAT " exited %s", pid,
+             g_spawn_check_wait_status (status, NULL) ? "normally" : "abnormally");
+
+  // Free any resources associated with the child here, such as I/O channels
+  // on its stdout and stderr FDs. If you have no code to put in the
+  // child_watch_cb() callback, you can remove it and the g_child_watch_add()
+  // call, but you must also remove the G_SPAWN_DO_NOT_REAP_CHILD flag,
+  // otherwise the child process will stay around as a zombie until this
+  // process exits.
+
+  g_spawn_close_pid (pid);
+}
+```
+
+## Spawn Functions
+
+ * [func@GLib.spawn_async_with_fds]
+ * [func@GLib.spawn_async_with_pipes]
+ * [func@GLib.spawn_async_with_pipes_and_fds]
+ * [func@GLib.spawn_async]
+ * [func@GLib.spawn_sync]
+ * [func@GLib.spawn_check_wait_status]
+ * [func@GLib.spawn_check_exit_status]
+ * [func@GLib.spawn_command_line_async]
+ * [func@GLib.spawn_command_line_sync]
+ * [func@GLib.spawn_close_pid]
+