docs: add basic LEDGE integration documentation

Add documentation describing the layers from LEDGE.

Signed-off-by: Marta Rybczynska marta.rybczynska@linaro.org

docs/supported-technologies/ledge.rst

+.. SPDX-FileCopyrightText: Linaro Ltd
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+.. include:: ../definitions.rst
+
+LEDGE (Experimental integration)
+################################
+
+The `LEDGE group from Linaro <https://www.linaro.org/automotive-iot-and-edge-devices/>`__
+is providing a set of reference layers for features as dependable boot, image signing
+and more. |main_project_name| is integrating Yocto Project/OpenEmbedded layers provided
+by the group. Please note that currently |main_project_name| is providing forks of the
+work with some changes required by |main_project_name| that have not been upstreamed yet.
+
+The layers are included by default in all |main_project_name|, while a selected set
+of features is enabled.
+
+
+meta-ts
+*******
+
+The `meta-ts layer <https://gitlab.com/Linaro/trustedsubstrate/meta-ts>`__ provides
+Yocto Project/OpenEmbedded packaging of the
+`Trusted Substrate <https://linaro.atlassian.net/wiki/spaces/TS/overview>`
+providing firmware for a selected set of boards, with features like UEFI secure boot,
+measured boot with TPM, and dual-banked firmware update with bricking protection.
+
+You can learn more about `meta-ts` features from the
+`documentation <https://trs.readthedocs.io/en/latest/firmware/index.html>`__.
+
+meta-ledge-secure
+*****************
+
+The `meta-ledg-secure layer <https://gitlab.com/Linaro/trustedsubstrate/meta-ledge-secure>`
+provides features like kernel and module signing, configuration of SELinux, software
+modules needed to support TPMs.
+
+|main_project_name| currently uses the OPTEE and TPM support packaging from
+`meta-ledge-secure`. Other features will be added in the near future.
+
+Other security layers
+*********************
+
+`meta-ts` and `meta-ledge-secure` depend on a number of other security layers from the
+industry, like `meta-secure-core` and `meta-security`. All those layers are included
+in the default build of |main_project_name|.