docs: add explanation on how to use custom APT configuration

[renanrodrigo]

Why is this needed?

This PR solves all of our problems because it documents how to use custom APT configuration in the Pro Client.

[renanrodrigo]

@s-makin all addressed, thanks for looking!

Link check fails because the security CVE endpoints are so nice

[renanrodrigo]

@orndorffgrant I made the edit to the python code - added a try...finally block there to the example, let me know if you think it looks better