Domains: show amount available and limit creation

[humitos]

I do think we want to add the domain count somewhere in the dashboard, though I feel like we need an "Organization status" page or something, but probably start on the Domains page for the project?

(from https://github.com/readthedocs/website/pull/305#pullrequestreview-2145935045)

Reference:

• https://github.com/readthedocs/readthedocs-corporate/issues/1808
• https://github.com/readthedocs/website/pull/305
• https://github.com/readthedocs/ext-theme/issues/321