Add section about using TLS with custom CA in regular build scripts

What does this MR do?

This MR adds documentation regarding how to proceed when a user needs to leverage a custom CA root certificate in a regular build script (e.g. to do a secondary Git clone, or fetch a file through HTTPS).

Why was this MR needed?

See #3748 (closed), #3417 (closed) and #3371 (closed)

Are there points in the code the reviewer needs to double check?

I've verified that these steps work as expected and I can perform a wget on the local HTTPS GitLab server from within the user build script.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

#3748 (closed) #3371 (closed) #3417 (closed)