Some more changes to the installation instructions and config files.
The contents of docs/vexim-acl-check-spf.conf seems to be obsolete since Exim v4.52, so I'm removing it and instead adding a commented-out block to docs/vexim-acl-check-rcpt.conf.
I've consolidated attachment extension filters in docs/vexim-acl-check-content.conf into one.
I'm also getting rid of the MY_IP macro. It was only used twice in the configuration, and I think we can avoid it in both cases.
Also, a bunch of Debian-specific files have been added to docs/debian-conf.d/, along with some instructions on how to use them. Unfortunately, I lack knowledge/understanding to make this Debian integration really perfect, but this is a start.