docs: clarify a custom domain belongs to one docs project

[devin-ai-integration]

Summary

Documents custom-domain ownership semantics, which weren't stated anywhere: a custom domain serves whichever docs deployment published most recently with that domain declared, and uniqueness is only enforced across organizations. Two repos in the same org can both declare one domain and silently overwrite each other on publish.

• setting-up-your-domain.mdx: new ### One domain per docs project section (with the multi-source exception called out) plus a <Warning> covering the same-org collision and the fix (remove the domain from all but the owning repo, republish).
• site-level-settings.mdx: one-sentence pointer from the instances.custom-domain ParamField to that anchor.

Vale hook passes; fern check --warnings reports 0 errors.

Link to Devin session: https://app.devin.ai/sessions/b9be75edfa5044d4868aabbc3b93ca4c

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785769761-custom-domain-uniqueness.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/configuration/site-level-settings
• learn/docs/preview-publish/setting-up-your-domain