Move index.php to index.html

It doesn't contain any PHP code, and this way PHP doesn't have to
process the main page, and we can serve it as a static asset, being
able also to better cache it.

Change-Id: I5fb1b71d382c0686b062471d92035651d16266b0
diff --git a/index.php b/index.html
similarity index 100%
rename from index.php
rename to index.html