doc: Move bootstd into its own directory

Before adding more files, move the bootstd docs into a new directory,
with an index.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/doc/usage/cmd/bootflow.rst b/doc/usage/cmd/bootflow.rst
index 6519e48..5d41fe3 100644
--- a/doc/usage/cmd/bootflow.rst
+++ b/doc/usage/cmd/bootflow.rst
@@ -26,7 +26,7 @@
 The `bootflow` command is used to manage bootflows. It can scan bootdevs to
 locate bootflows, list them and boot them.
 
-See :doc:`../../develop/bootstd` for more information.
+See :doc:`/develop/bootstd/index` for more information.
 
 Note that `CONFIG_BOOTSTD_FULL` (which enables `CONFIG_CMD_BOOTFLOW_FULL) must
 be enabled to obtain full functionality with this command. Otherwise, it only