Add versions.md and version.h files describing version branches and tags

[ryanofsky]

Having libmultiprocess version numbers should be helpful for:

• Backporting fixes for previous Bitcoin Core releases if we need to do that at some point (How to backport libmultiprocess changes? bitcoin/bitcoin#33439).
• Letting Bitcoin Core releases describe what libmultiprocess versions they are compatible with (29.x: using a local libmultiprocess install will no-longer work bitcoin/bitcoin#33576)
• Letting code detect what version is used at compile-time or runtime, for logging, feature detection, error reporting, or other purposes.

[DrahtBot]

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ACK	TheCharlatan

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LLM Linter (✨ experimental)

Possible typos and grammar issues:

• include -> header [Sentence fragment: "Versioning policy is described in the version.h include." — "include" is awkward/incorrect here; "header" or "include file" makes the meaning clear.]

^{drahtbot_id_5_m}

[ryanofsky]

Updated 2b0dd92 -> 6e01d2d (pr/versions.1 -> pr/versions.2, compare) fixing llm linter errors

[TheCharlatan]

ACK 6e01d2d