add ref to howto guide in pypi docs

Author: rickeylev

docs/pypi/index.md

@@ -11,6 +11,7 @@ Using PyPI packages (aka "pip install") involves the following main steps:
 
 With the advanced topics covered separately:
 * Dealing with [circular dependencies](./circular-dependencies).
+* Handling [multi-platform dependencies](../howto/multi-platform-pypi-deps).
 
 ```{toctree}
 lock
@@ -22,6 +23,9 @@ use
 ## Advanced topics
 
 ```{toctree}
+:maxdepth: 1
+
 circular-dependencies
 patch
+../howto/multi-platform-pypi-deps
 ```

python/private/pypi/pip_repository.bzl

@@ -266,6 +266,9 @@ code will be re-evaluated when any of files in the default changes.
 Those dependencies become available in a generated `requirements.bzl` file.
 You can instead check this `requirements.bzl` file into your repo, see the "vendoring" section below.
 
+For advanced use-cases, such as handling multi-platform dependencies, see the
+[How-to: Multi-Platform PyPI Dependencies guide](/howto/multi-platform-pypi-deps).
+
 In your WORKSPACE file:
 
 ```starlark