Auto merge of rust-lang#84583 - joshtriplett:target-tier-policy, r=pietroalbini

Add the target tier policy from accepted RFC 2803

Author: bors

src/doc/rustc/src/SUMMARY.md

@@ -13,6 +13,7 @@
 - [JSON Output](json.md)
 - [Tests](tests/index.md)
 - [Platform Support](platform-support.md)
+- [Target Tier Policy](target-tier-policy.md)
 - [Targets](targets/index.md)
     - [Built-in Targets](targets/built-in.md)
     - [Custom Targets](targets/custom.md)

src/doc/rustc/src/platform-support.md

@@ -7,7 +7,8 @@
 </style>
 
 Support for different platforms are organized into three tiers, each with a
-different set of guarantees.
+different set of guarantees. For more information on the policies for targets
+at each tier, see the [Target Tier Policy](target-tier-policy.md).
 
 Platforms are identified by their "target triple" which is the string to
 inform the compiler what kind of output should be produced. The columns in the