Docs: expand "Custom domains" to mention subprojects

[humitos]

We should communicate to our users that before adding another custom domain for their project, they should probably consider using subprojects. This would avoid having:

• users.docs.company.com
• sales.docs.company.com
• tech.docs.company.com

There were some companies that followed the approach to add a custom domain we have documented at https://docs.readthedocs.io/en/stable/guides/custom-domains.html but they only needed to follow that guide only once and then use subprojects for the other projects.

We should probably promote using docs.company.com as the main documentation and then use a subproject for the others, exposing them at docs.company.com/projects/users/ instead.

I'm writing this issue based on user feedback from a few customers.

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