Add ReScript-React migration guide (#267)

Author: chenglou

data/sidebar_react_latest.json

@@ -1,7 +1,8 @@
 {
   "Overview": [
     "introduction",
-    "installation"
+    "installation",
+    "migrate-from-reason-react"
   ],
   "Main Concepts": [
     "elements-and-jsx",

pages/docs/react/latest/installation.mdx

@@ -6,9 +6,9 @@ canonical: "/docs/react/latest/installation"
 
 # Installation
 
-**Requirements:** 
+**Requirements:**
 
-- `[email protected]` or later 
+- `[email protected]` or later
 - `[email protected]` or later
 
 Add following dependency to your ReScript project (in case you don't have any project yet, check out the [installation instructions](/docs/manual/latest/installation) in the manual):
@@ -38,7 +38,7 @@ let make = () => {
 
 Now run `npx bsb -make-world` and you should see a successful build.
 
-## Exposed Modules 
+## Exposed Modules
 
 After a successful installation, `@rescript/react` will make following modules available in your project's global scope:
 
@@ -48,12 +48,3 @@ After a successful installation, `@rescript/react` will make following modules a
 - `ReactEvent`: Bindings to React's synthetic events
 - `ReactDOMStyle`: Bindings to the inline style API
 - `RescriptReactRouter`: A simple, yet fully featured router with minimal memory allocations
-
-
-### Deprecated Modules
-
-`rescript-react` has gone a long way, so we still keep around a few modules for legacy reasons. We will deprecate them in the future in favor of...
-
-- `ReactDOMRe` -> `ReactDOM`
-
-Actual usage and for each module will be discussed in the rest of this documentation.

pages/docs/react/latest/migrate-from-reason-react.mdx

@@ -0,0 +1,19 @@
+---
+title: Migrate from ReasonReact
+description: "Migrate from ReasonReact"
+canonical: "/docs/react/latest/migrate-from-reason-react"
+---
+
+# Migrate from ReasonReact
+
+ReScript-React is mostly feature-compatible with ReasonReact, with a few long deprecated modules now removed:
+
+- `ReactEventRe`: use `ReactEvent`.
+- `ReactDOMServerRe`: use `ReactDOMServer`.
+- `ReactDOMRe`: use `ReactDOM`.
+- `ReasonReact`: use `React`.
+- `ReasonReactCompat` fully removed. Was [used](https://reasonml.github.io/reason-react/docs/en/reasonreactcompat) to migrate to ReasonReact `0.7.0`.
+
+Reminder: for `ReactDOMRe.renderToElementWithClassName` and `ReactDOMRe.renderToElementWithId`, use `ReactDOM.querySelector` + `ReactDOM.render` if you haven't moved to `ReactDOM` already previously.
+
+Note: to migrate to ReScript-React, you need to be on ReasonReact `>=0.8.0`. If not, please first [upgrade to the latter](https://github.com/reasonml/reason-react/blob/master/HISTORY.md#080-042020). Sorry for the trouble.