doc: android: Convert to Sphinx format

Convert Android documentation from regular txt format to Sphinx (RST).
Also add Android index.rst file and reference it in root index.rst, so
that Android documentation is visible.

Test:

    $ make htmldocs
    $ xdg-open doc/output/index.html

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 6a2f6fb..e6ba570 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -880,7 +880,7 @@
 	  Android devices. Fastboot requires either the network stack
 	  enabled or support for acting as a USB device.
 
-	  See doc/android/fastboot.txt for more information.
+	  See doc/android/fastboot.rst for more information.
 
 config CMD_FLASH
 	bool "flinfo, erase, protect"