docs: document the chained Cloudflare custom-domain topology

João Pereirarequested to merge
jdrpereira/docs-cf-chained-topology into main

Updates docs/dev/gitlab-com-infrastructure.md to describe the chained Cloudflare custom-domain setup (child zone -> Runway/SaaS zone -> origin) that replaced the plain custom hostnames. Adds an ASCII diagram, the shared edge settings (max upload, unbuffered uploads, read timeout, cache bypass), the zones, and where each piece is managed.

Docs-only, no behavior change.

Edited by João Pereira

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