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/develop/index.rst b/doc/develop/index.rst
index f9c4bf8..c0107a7 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -29,7 +29,7 @@
 
    directories
    bloblist
-   bootstd
+   bootstd/index
    ci_testing
    commands
    config_binding