Add link from api-docs to namespace docs

[mladlow]

Description

I wanted to make the list of API endpoints with restricted namespace access easier to find.

I considered adding the partial directly here, but thought the explanation in the link would have been helpful.

I personally was trying to find this table and thought the API docs seemed like a natural place to look for it, but it wasn't there.

TODO only if you're a HashiCorp employee

• Labels: If this PR is the CE portion of an ENT change, and that ENT change is
  getting backported to N-2, use the new style backport/ent/x.x.x+ent labels
  instead of the old style backport/x.x.x labels.
• Labels: If this PR is a CE only change, it can only be backported to N, so use
  the normal backport/x.x.x label (there should be only 1).

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