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/test/py/tests/test_android/test_avb.py b/test/py/tests/test_android/test_avb.py
index 20ccaf6..a04a7ff 100644
--- a/test/py/tests/test_android/test_avb.py
+++ b/test/py/tests/test_android/test_avb.py
@@ -8,7 +8,7 @@
 This tests Android Verified Boot 2.0 support in U-boot:
 
 For additional details about how to build proper vbmeta partition
-check doc/android/avb2.txt
+check doc/android/avb2.rst
 
 For configuration verification:
 - Corrupt boot partition and check for failure