docs: perf: Add imx-ddr to documentation index

Sphinx is currently outputting a warning where
the file 'imx-ddr.rst' is not included in the
documentation index. Additionally, the code
highlighting and doc formatting can be slightly
improved.

Signed-off-by: Adam Zerella <adam.zerella@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>