AW docs: guide for using the WebView shell

This adds a developer guide for the WebView shell. This covers building
and installing, but also shows how to interact with the shell via the
commandline.

This cross-links to the shell's folder (adding a README.md), but keeps
primary documentation under android_webview/docs/ to improve
discoverability.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I6216bb70a42989910044cfc56e56314a4e2bc781
Reviewed-on: https://chromium-review.googlesource.com/c/1490909
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636952}

android_webview/docs/navbar.md

@@ -3,6 +3,7 @@
 * [Home][home]
 * [Device Setup](/android_webview/docs/device-setup.md)
 * [Test instructions](/android_webview/docs/test-instructions.md)
+* [WebView Shell](/android_webview/docs/webview-shell.md)
 * [Commandline flags](/android_webview/docs/commandline-flags.md)
 * [Net debugging](/android_webview/docs/net-debugging.md)
 

android_webview/docs/webview-shell.md

+# System WebView Shell
+
+WebView team maintains a "shell"--a thin interface over the WebView APIs--to
+exercise WebView functionality. The System WebView Shell (AKA "shell browser,"
+"WebView shell") is a standalone app implemented [in
+chromium](/android_webview/tools/system_webview_shell/). While often used for
+manual testing, we also use the shell for automated tests (see our [layout and
+page cycler tests](./test-instructions.md#layout-tests-and-page-cycler-tests)).
+
+*** note
+This relies on the WebView installed on the system. So if you're trying to
+verify local changes to WebView, or run against a specific WebView build, you
+must **install WebView first.**
+***
+
+*** promo
+**Tip:** the shell displays the WebView version (the corresponding [chromium version
+number](https://www.chromium.org/developers/version-numbers)) in the title bar
+at the top. This can be helpful for checking which WebView version is installed
+& selected on the device.
+***
+
+## Working with the shell
+
+```sh
+# Build and install
+$ autoninja -C out/Default system_webview_shell_apk
+$ out/Default/bin/system_webview_shell_apk install
+
+# Launch a URL from the commandline, or open the app from the app launcher
+$ out/Default/bin/system_webview_shell_apk launch "https://www.google.com/"
+
+# For more commands:
+$ out/Default/bin/system_webview_shell_apk --help
+```

android_webview/tools/system_webview_shell/README.md

+# WebView Shell
+
+The WebView shell is a standalone application for testing WebView. For more
+information, please see [the
+documentation](/android_webview/docs/webview-shell.md).