docs: link to readthedocs instead of docs directory in readme

The docs directory doesn't work anymore; readthedocs is where the user-facing
docs are now at

PiperOrigin-RevId: 540283201

README.md

@@ -1,9 +1,9 @@
 [![Build
 status](https://badge.buildkite.com/a82ebafd30ad56e0596dcd3a3a19f36985d064f7f7fb89e21e.svg?branch=master)](https://buildkite.com/bazel/rules-testing)
 
-# Framworks and utilities for testing Bazel Starlark rules
+# Framworks and utilities for testing Bazel Starlark
 
-`rules_testing` provides frameworks and utilities to make testing Starlark rules
+`rules_testing` provides frameworks and utilities to make testing Starlark code
 easier and convenient.
 
-For detailed docs, see the [docs directory](docs/index.md).
+For detailed docs, see the [docs website](https://rules-testing.readthedocs.io)