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/MAINTAINERS b/MAINTAINERS
index f995394..a6e47e8 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -927,7 +927,7 @@
 F:	boot/bootstd.c
 F:	cmd/bootdev.c
 F:	cmd/bootflow.c
-F:	doc/develop/bootstd.rst
+F:	doc/develop/bootstd/
 F:	doc/usage/bootdev.rst
 F:	doc/usage/bootflow.rst
 F:	doc/usage/bootmeth.rst