docs: convert cast build instructions from html to markdown

docs/cast_build_instructions.md

+# Cast Build Instructions
+
+**Note**: it is **not possible** to build a binary functionally
+equivalent to a Chromecast. This is to build a single-page content
+embedder with similar functionality to Cast products.
+
+## Prerequisites
+
+*   See the [Linux build prerequisites](https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions_prerequisites.md)
+
+## Setting Up
+
+*   Cast Linux build only: [Linux build
+    setup](https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions.md)
+    is sufficient.
+*   Cast Linux and Android builds: follow the [Android build
+    setup](https://www.chromium.org/developers/how-tos/android-build-instructions)
+    instructions.
+
+## Building/running cast\_shell (Linux)
+
+```shell
+gn gen out/Debug --args='is_chromecast=true is_debug=true'
+ninja -C out/Debug cast_shell
+```
+
+```shell
+out/Debug/cast_shell --ozone-platform=egltest http://google.com
+```
+
+## Building/running cast\_shell\_apk (Android)
+
+```shell
+gn gen out/Debug --args='is_chromecast=true target_os="android" is_debug=true'
+ninja -C out/Debug cast_shell_apk
+```
+
+```shell
+adb install out/Debug/apks/CastShell.apk
+adb shell am start -d "http://google.com" org.chromium.chromecast.shell/.CastShellActivity
+```