[Magic Transit] Use RFC 5737 prefixes in route examples

[SomSamantray]

Summary

Five Magic Transit pages used non-reserved addresses as placeholders for a customer's onboarded or advertised prefix. Magic Transit routes public prefixes only, and the style guide already mandates the RFC 5737 blocks for IPv4 examples, so the examples were both impossible as written and inconsistent with the rest of the docs.

Two kinds of placeholder were wrong: RFC 1918 private addresses, and — on two pages — subnets of Cloudflare's own published 103.21.244.0/22 range presented as if they were a customer's prefixes. Both now use reserved documentation space. reference/traffic-steering.mdx already carried an Aside stating that these blocks are reserved for documentation examples, so this change makes that page consistent with its own note.

The bandwidth-measurement example keeps its Longest Prefix Match shape. RFC 5737 reserves only /24 blocks, so its covering prefix and the more-specific ISP route become 203.0.113.0/24 and 203.0.113.128/25, and the prose reference to the covering /20 becomes /24.

Addresses that are factual references to real infrastructure are deliberately unchanged — Cloudflare's tunnel health-check source range, tunnel-interface addresses, documented CGNAT space, and a captured traceroute's hops would all become untrue if converted.

One non-reserved prefix is deliberately left alone: the /24-into-/23 summarization example at reference/traffic-steering.mdx:225-228 uses 192.0.3.0/24. RFC 5737's three blocks are not contiguous, so no reserved-space pair can be summarized into a /23, and the same page documents a /24 minimum advertising prefix, which rules out a sub-/24 rewrite. Removing it means reshaping the example, which is a separate editorial change. The Aside just below that example also claims RFC 5737 coverage for 192.0.3.0/24, which is inaccurate.

Fixes #31625

Documentation checklist

• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

[cloudflare-docs-bot]

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