bpo-43987: Add "Annotations Best Practices" HOWTO doc. (#25746)

Add "Annotations Best Practices" HOWTO doc.
diff --git a/Doc/howto/index.rst b/Doc/howto/index.rst
index e0dacd2..eae8f14 100644
--- a/Doc/howto/index.rst
+++ b/Doc/howto/index.rst
@@ -30,4 +30,5 @@
    ipaddress.rst
    clinic.rst
    instrumentation.rst
+   annotations.rst