docs: deprecated.md to sphinx docs (#6454)

Moved DEPRECATED.md to sphinx docs.

Risk Level: Low - only documentation
Testing: Compiles with sphinx docs without warnings or errors
Docs Changes: deprecated.rst created in intro section of sphinx docs and added to toctree
Release Notes: N/A

Fixes: #6386

Signed-off-by: HashedDan <georgedanielmangum@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ fd7c172af181275693297efbe148fd8bb414ef48

envoy/api/v2/cds.proto

@@ -194,7 +194,7 @@ message Cluster {
   // :ref:`STRICT_DNS<envoy_api_enum_value_Cluster.DiscoveryType.STRICT_DNS>`
   // or :ref:`LOGICAL_DNS<envoy_api_enum_value_Cluster.DiscoveryType.LOGICAL_DNS>` clusters.
   // This field supersedes :ref:`hosts<envoy_api_field_Cluster.hosts>` field.
-  // [#comment:TODO(dio): Deprecate the hosts field and add it to DEPRECATED.md
+  // [#comment:TODO(dio): Deprecate the hosts field and add it to :ref:`deprecated log<deprecated>`
   // once load_assignment is implemented.]
   //
   // .. attention::