AW docs: add more to README

No change to logic, only docs.

This expands on //android_webview/README.md, taking some content from
//android_webview/docs/README.md. I'm taking this opportunity to update
links to provide better context (ex. link to the package-summary rather
than individual frameworks classes).

The distinction between these two README files is:

* //aw/README.md describes WebView overall
* //aw/docs/README.md describes how to work with WebView in chromium

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I89fb481b16c1fd7258fb409c6d61989497573182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807656Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696964}

android_webview/README.md

 # Android WebView
 
-This directory contains the Android WebView implementation.
+Android WebView is an Android system component for displaying web content.
+[WebView](https://developer.android.com/reference/android/webkit/WebView) (and
+the [related Android classes][1]) are implemented by the code in the
+`//android_webview/` folder.
 
-See https://developer.android.com/reference/android/webkit/WebView for
-developer documentation.
+Android WebView is a content embedder, meaning it depends on code in
+`//content/` and lower layers (ex. `//net/`, `//base/`), but does not depend on
+sibling layers such as `//chrome/`.
+
+This directory contains the Android WebView implementation, as well as the
+implementation for the [AndroidX Webkit support library][2].
+
+## Want to use WebView in an Android app?
+
+Please consult our API documentation and app development guides:
+
+* [Android Frameworks classes][1]
+* [AndroidX Webkit support library][2]
+* [Development guides](https://developer.android.com/guide/webapps)
+
+## Want to build and install WebView on a device?
+
+See our [Chromium developer documentation](docs/README.md).
+
+[1]: https://developer.android.com/reference/android/webkit/package-summary
+[2]: https://developer.android.com/reference/androidx/webkit/package-summary

android_webview/docs/README.md

@@ -13,18 +13,8 @@ Please see the markdown files in this directory for detailed developer guides.
 
 ## What is Android WebView?
 
-Android WebView is an Android system component for displaying web content. This
-(and the related Android classes) are implemented by the code in the
-`//android_webview/` folder.
-
-Android WebView is a content embedder, meaning it depends on code in
-`//content/` and lower layers (ex. `//net/`, `//base/`), but does not depend on
-sibling layers such as `//chrome/`.
+See [//android\_webview/README.md](/android\_webview/README.md).
 
 ## Want to use WebView in an Android app?
 
-Please consult https://developer.android.com/reference/android/webkit/WebView,
-and the related classes in the `android.webkit` package. You may also be
-interested in our
-[`androidx.webkit`](https://developer.android.com/reference/androidx/webkit/WebViewCompat)
-API documentation.
+See [//android\_webview/README.md](/android\_webview/README.md).