logs: move appendices Data Model Appendix document (#3207)

Fixes #3206 

## Changes

This PR moves Data Model Appendix A and Appendix B to a separate
document, disentangling these appendices from the stability of the Data
Model document itself.

---------

Co-authored-by: Tigran Najaryan <[email protected]>

Author: braydonk

CHANGELOG.md

@@ -174,6 +174,8 @@ release.
 
 - Clarify usage of log body for structured logs
   ([#3023](https://github.com/open-telemetry/opentelemetry-specification/pull/3023))
+- Move appendices from Data Model to new Data Model Appendix document
+  ([#3207](https://github.com/open-telemetry/opentelemetry-specification/pull/3207))
 
 ### Resource
 

specification/logs/README.md

@@ -138,7 +138,7 @@ Given the above state of the logging space we took the following approach:
   OpenTelemetry's log data model. More on this [later](#new-first-party-application-logs).
 
 - Existing log formats can be
-  [unambiguously mapped](data-model.md#appendix-a-example-mappings) to
+  [unambiguously mapped](data-model-appendix.md) to
   OpenTelemetry log data model. OpenTelemetry Collector can read such logs and
   translate them to OpenTelemetry log data model.