Docs: Clearly document where we do and do not provide API stability gaurantees

> [!NOTE]
> Migrated from [augurlabs/augur#3673](https://github.com/augurlabs/augur/issues/3673)
> Originally opened by `@MoralCode` on 2026-02-10

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I think it would be helpful for us as a project to be clear about what parts of augurs various APIs (intended or not) can be expected to remain stable (or at least have some notice time/deprecation window before things change) and which may change without warning

This will help make sure we dont break peoples workflows while still allowing forward progress as was discussed during CHAOSScon EU 2026