Rework documentation

Move the documentation to a common "doc" folder to make docs more
discoverable.

Split documentation into multiple pages.

Separate Android & Linux build documentation. Android is the primary
platform so the build docs now prioritises instructions for Android.

Add patch as build prerequisite.

Move build.sh documentation inside the script itself. Otherwise remove
it from the documentation as it has been found to create more confusion
than it solves except for advanced users.

Mark KLEIDICV_CHECK_BANNED_FUNCTIONS as internal since there's no
particular need for users to touch it.