docs: add ASAM OpenX standards reference library as submodule (#78)

Add ASCS-eV/asam-openx-standards as a submodule at submodules/asam-openx-standards.
Contains version-pinned markdown copies of all publicly available ASAM OpenX
simulation standards, redistributed under the ASAM Unrestricted Distribution Clause.

Standards included (251 chapter files across 8 domains):
- ASAM OpenDRIVE v1.9.0 (95 chapters)
- ASAM OpenSCENARIO DSL v2.2.0 (59 chapters)
- ASAM OpenODD v1.0.0 (62 chapters)
- ASAM OpenMATERIAL 3D BS 1.0.0 (27 chapters)
- ASAM OpenLABEL v1.0.0 (overview + JSON schema reference)
- ASAM OSI (nested submodule, MPL-2.0)
- ASAM OpenCRG (nested submodule, Apache-2.0)
- ASAM TrafficParticipants v1.0.2 (overview only)
- ISO 34503 (paraphrased summary)

Legal basis: each ASAM OpenX standard carries an Unrestricted Distribution
exception replacing the restrictive section 2(1) of the ASAM license, granting everyone
a basic, non-exclusive and unlimited license to use the standard.

Signed-off-by: Carlo van Driesten <carlo.van-driesten@bmw.de>

Author: jdsika

.gitmodules

@@ -10,3 +10,6 @@
 [submodule "submodules/linkml"]
 	path = submodules/linkml
 	url = https://github.com/ASCS-eV/linkml.git
+[submodule "submodules/asam-openx-standards"]
+	path = submodules/asam-openx-standards
+	url = https://github.com/ASCS-eV/asam-openx-standards.git