DOC: adding artefact retention policy to SPEC4

[bsipocz]

This is to address #261
I'm on the opinion that it was a mistake to include so much implementation detail in the SPEC itself, but as the other specifics of https://anaconda.org/scientific-python-nightly-wheels are in the docs, we should list these policies, too.

Also note, that we may eventually change the actual numbers, e.g. scientific-python/upload-nightly-action#35

cc @mattip

[mattip]

Thanks, LGTM (I had to look up artefact to discover it is the British spelling of the American artifact)

[jarrodmillman]

Should we leave the actual numbers out of the SPEC? I think it is sufficient to mention there is a limit and that you can find it in the action. Just trying to keep the actual SPEC documents short and reduce the amount of places we need to update things if they change.

[bsipocz]

Should we leave the actual numbers out of the SPEC? I think it is sufficient to mention there is a limit and that you can find it in the action. Just trying to keep the actual SPEC documents short and reduce the amount of places we need to update things if they change.

Yep, I like that, and will update it in the text.