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>
10 files changed