Consider moving channel_convert_to_unsigned out of detail namespace

From @marco-langer https://github.com/boostorg/gil/pull/703#issuecomment-1172915455

> Regarding this channel_convert_to_unsigned, which is now in the detail namespace, but also still referenced in this tutorial:
>
> Maybe one should think to pull it out of the detail namespace again, if it is that useful for client code? I don't know the rational why it was moved into the detail namespace at all (this was done 15 years ago!).

I think, it is worth to keep this discussion open and come back to it in future.

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For the records, it moved into `detail` in https://github.com/boostorg/gil/commit/2ad6747dd0472cef7c67b3438b847f347ce45212
which refers to the "GIL 2.0 to GIL 2.1 Changes" PDF. Since I have got this [gil2.1_changes.pdf](https://github.com/boostorg/gil/files/9041592/gil2.1_changes.pdf), I've attached it for the reference. It does not explain the move rationale though.

I attached also earlier [gil2.0_changes.pdf](https://github.com/boostorg/gil/files/9041602/gil2.0_changes.pdf) which lists lots of changes related to Boost'ification of the Adobe GIL library. This makes me guess that moving of the types into the `detail` namespace may be related to the unification of GIL according to Boost conventions.

(both legacy documents available at https://github.com/boostorg/gil/wiki/History)