Create spaces-custom-domain.md

[Michellehbn]

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[HuggingFaceDocBuilderDev]

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[burtenshaw]

LGTM

[abidlabs]

Can we merge this in? I was looking for information about custom domains on Spaces and couldn't find it.

Also: as it turns out, if you add a custom domain to your Space, the SPACE_HOST env variable becomes a comma separated list that includes both the original hf.space domain and the custom domain. Would be good to note that either in this doc or in https://github.com/huggingface/hub-docs/edit/main/docs/hub/spaces-overview.md

[pcuenca]

Can we merge this in? I was looking for information about custom domains on Spaces and couldn't find it.

Good point @abidlabs! I think @Michellehbn is OOO, so I'll take the liberty to accept all suggestions and merge. My apologies Michelle! 🙏

Regarding your additional suggestion, in my opinion we can iterate in a new PR.

[julien-c]

needs to be added to the TOC otherwise users can't find it