Document Domain Research access, usage visibility, and monthly cap

[dallasread]

Tightens content/v2/domains/research.md so a developer reading the page
can tell, without leaving it, that they need the Domain Research
subscription and the domain_research_read scope, where their
month-to-date usage shows up in-app, and what the cap-exceeded response
looks like.

The cap number is intentionally not published — only that a cap exists
and depends on the subscription. The shared /v2/#too-many-requests
reference already covers the body shape and X-RateLimit-* headers, so
the cap-specific Retry-After is documented in narrative only; see the
issue notes for the OpenAPI $ref reasoning.

Fixes #1021
Belongs to https://github.com/dnsimple/dnsimple-business/issues/2779

🔍 QA

• Render the Domain Research page locally
  • bundle install && yarn install
  • yarn live
  • Open http://localhost:3000/v2/domains/research/
  • Verify the new Access, Usage visibility, and Monthly request cap sections render under the existing private-beta banner.
  • Verify the HTTP 429 line in Errors links to http://localhost:3000/v2/#too-many-requests and the in-page link to Monthly request cap anchors correctly.

📋 Deployment Pre/Post tasks

N / A

Deployment Verification

• production: Verify a successful deploy

[lokst]

This looks great, thanks!