Improve TLS custom cert documentation

What does this MR do?

• It updates the tls-self-signed.md page to be much more explicit regarding the options available to the user regarding how to configure TLS, namely outlining 2 main scenarios: GitLab-only connections, and generic OS-level TLS configuration that works with other applications.
• It also outlines the two contexts that must be considered: build containers and helper containers. It explains why in the build container the user is responsible for all the TLS setup that might be required.
• It adds a note (with link to issue) regarding Kubernetes not respecting the image ENTRYPOINT, and therefore TLS certificates not being automatically installed.

Why was this MR needed?

See #27092

What's the best way to test this MR?

What are the relevant issue numbers?

#27092 #27067 (closed) #27055 (closed)