[compiler] Move React 17/18 section to its own subheading (#7230)

Currently it's a little hidden, let's move it to its own subheading for
more prominence

Author: poteto

src/content/learn/react-compiler.md

@@ -184,6 +184,31 @@ const ReactCompilerConfig = {
 
 When you have more confidence with rolling out the compiler, you can expand coverage to other directories as well and slowly roll it out to your whole app.
 
+### Using React Compiler with React 17 or 18 {/*using-react-compiler-with-react-17-or-18*/}
+
+React Compiler works best with React 19 RC. If you are unable to upgrade, you can install the extra `react-compiler-runtime` package which will allow the compiled code to run on versions prior to 19. However, note that the minimum supported version is 17.
+
+<TerminalBlock>
+npm install react-compiler-runtime@experimental
+</TerminalBlock>
+
+You should also add the correct `target` to your compiler config, where `target` is the major version of React you are targeting:
+
+```js {3}
+// babel.config.js
+const ReactCompilerConfig = {
+  target: '18' // '17' | '18' | '19'
+};
+
+module.exports = function () {
+  return {
+    plugins: [
+      ['babel-plugin-react-compiler', ReactCompilerConfig],
+    ],
+  };
+};
+```
+
 #### New projects {/*new-projects*/}
 
 If you're starting a new project, you can enable the compiler on your entire codebase, which is the default behavior.
@@ -369,16 +394,6 @@ To report issues, please first create a minimal repro on the [React Compiler Pla
 
 You can also provide feedback in the React Compiler Working Group by applying to be a member. Please see [the README for more details on joining](https://github.com/reactwg/react-compiler).
 
-### What does the compiler assume? {/*what-does-the-compiler-assume*/}
-
-React Compiler assumes that your code:
-
-1. Is valid, semantic JavaScript.
-2. Tests that nullable/optional values and properties are defined before accessing them (for example, by enabling [`strictNullChecks`](https://www.typescriptlang.org/tsconfig/#strictNullChecks) if using TypeScript), i.e., `if (object.nullableProperty) { object.nullableProperty.foo }` or with optional-chaining `object.nullableProperty?.foo`.
-3. Follows the [Rules of React](https://react.dev/reference/rules).
-
-React Compiler can verify many of the Rules of React statically, and will safely skip compilation when it detects an error. To see the errors we recommend also installing [eslint-plugin-react-compiler](https://www.npmjs.com/package/eslint-plugin-react-compiler).
-
 ### How do I know my components have been optimized? {/*how-do-i-know-my-components-have-been-optimized*/}
 
 [React Devtools](/learn/react-developer-tools) (v5.0+) has built-in support for React Compiler and will display a "Memo ✨" badge next to components that have been optimized by the compiler.