Add a support_policy.md

[AoifeHughes]

I noticed:
- https://github.com/chalk-lab/Mooncake.jl/blob/main/SUPPORT_POLICY.md
This could be something worth adding to the repo?

[penelopeysm]

@willtebbutt can confirm / deny, but I think the main reason why Mooncake has a policy that is so stringent is because it is quite closely tied to Julia compiler internals and that makes it very hard to fully support old patch versions of Julia (which may have bugs in the compiler) or multiple minor versions (which can have vastly breaking changes). Since Turing works at a much higher level, it has a correspondingly smaller dependence on the Julia version, so we haven't had a pressing need to set out our case so strongly.

In the absence of such a document, the support policy for Turing.jl would be defined by the version bounds in Project.toml (as of the time of writing, 1.10.2 and above). In CI, we test on min and 1, which respectively resolve to the lowest allowed version in Project.toml (i.e. 1.10.2), and the latest stable patch release of Julia (1.11.5 as of the time of writing). We can't verify, but we assume that it works for all versions in between.

[AoifeHughes]

Fair enough, I just thought it would be useful to collate everything that's supported