Documentation: Add descriptions for environment variables

Describe the environment variables used in libcamera, excluded
variables are `LIBCAMERA_IPA_FORCE_C_API` and `LIBCAMERA_IPA_PROXY_PATH`,
the former because it is likely to be removed and the later because
it has no current use-case.

Add a brief explanation for the IPA configuration and IPA modules.
List all the available Log levels and categories and add a short guide
on how to use them for debugging.

Signed-off-by: Sebastian Fricke <sebastian.fricke.linux@gmail.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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