Add design docs for versioning export

ament_cmake_core/doc/versioning.md

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+## Express Version Compatibility in the Installation
+
+Add a COMPATIBILITY argument to `ament_package`. Default to AnyNewerVersion if not supplied which matches current behavior.
+
+User call in `foo's` `CMakeLists.txt`:
+```cmake
+ament_package(COMPATIBILITY SameMajorVersion)
+```
+
+The compatibility would be forwarded to `write_basic_package_version_file`.
+
+Then, you can use it in `bar`
+```cmake
+find_package(foo_core 4 CONFIG REQUIRED)
+```
+
+If you forgot to update `foo_core` from 3 to 4 and rebuild your workspace, then colcon 
+will fail at the configure stage of `bar` with a nice error message.
+

ament_cmake_export_dependencies/doc/dependency_versioning_export.md

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+# Express Version Compatibility in Dependencies
+
+`ament_export_dependencies` shall propagate the version requested of dependencies to a call to `find_dependency <version>`.
+
+For example:
+`find_package(foo_core CONFIG 4)` results in
+`add_dependency(foo_core CONFIG 4)`. 
+
+`find_package(foo_core CONFIG 4.8.3-dev2)` results in
+`add_dependency(foo_core CONFIG 4.8.3-dev2)`. 
+
+`find_package(foo_core MODULE)` results in
+`add_dependency(foo_core MODULE)`. 
+
+I think this can be done automatically with the use of `foo_core_FIND_VERSION`.
+
+Adding in the find method of MODULE/CONFIG may be out of scope for this discussion.
+
+I want to avoid a user manually maintaining versions in multiple places.
+It's hard enough as it is to remember when you add a find_package call to also add it to `ament_export_dependencies`.