vbe: Add some documentation

Add a few links to documents about Verified Boot for Embedded (VBE).
These will be expanded as development proceeds.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/bootstd.rst

@@ -32,6 +32,7 @@ way to boot with U-Boot. The feature is extensible to different Operating
 Systems (such as Chromium OS) and devices (beyond just block and network
 devices). It supports EFI boot and EFI bootmgr too.
 
+Finally, standard boot supports the operation of :doc:`vbe`.
 
 Bootflow
 --------

doc/develop/index.rst

@@ -39,6 +39,7 @@ Implementation
    smbios
    spl
    uefi/index
+   vbe
    version
 
 Debugging

doc/develop/vbe.rst

@@ -0,0 +1,26 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Verified Boot for Embedded (VBE)
+================================
+
+Introduction
+------------
+
+VBE provides a standard boot mechanism for embedded systems. If defines
+how firmware and Operating Systems are located, updated and verified.
+
+Within U-Boot, one or more VBE bootmeths implement the boot logic. For example,
+the vbe-simple bootmeth handles finding the firmware (e.g. in MMC) and starting
+it. Typically the bootmeth is started up in VPL and controls which SPL and
+U-Boot binaries are loaded.
+
+A 'vbe' command provides access to various aspects of VBE's operation, including
+listing methods and getting the status for a method.
+
+For a detailed overview of VBE, see vbe-intro_. A fuller description of
+bootflows is at vbe-bootflows_ and the firmware-update mechanism is described at
+vbe-fwupdate_.
+
+.. _vbe-intro: https://docs.google.com/document/d/e/2PACX-1vQjXLPWMIyVktaTMf8edHZYDrEvMYD_iNzIj1FgPmKF37fpglAC47Tt5cvPBC5fvTdoK-GA5Zv1wifo/pub
+.. _vbe-bootflows: https://docs.google.com/document/d/e/2PACX-1vR0OzhuyRJQ8kdeOibS3xB1rVFy3J4M_QKTM5-3vPIBNcdvR0W8EXu9ymG-yWfqthzWoM4JUNhqwydN/pub
+.. _vbe-fwupdate: https://docs.google.com/document/d/e/2PACX-1vTnlIL17vVbl6TVoTHWYMED0bme7oHHNk-g5VGxblbPiKIdGDALE1HKId8Go5f0g1eziLsv4h9bocbk/pub