docs(use-cases): add Use Cases section with paid-API-key pattern

[r-marques]

Summary

New top-level Use Cases group at the top of the Guides tab in docs.json, with four new pages framing productized integration patterns rather than feature tours.

The new section's job is to answer "is this the right shape for what I'm building?" before a reader dives into the technical Solutions pages. Per discussion with @r-marques on nevermined-io/nvm-monorepo#1684, this PR is a light-layer addition — the hub page cross-links into solutions/api-providers.mdx for the implementation deep-dive rather than duplicating it.

Second of three PRs landing nevermined-io/nvm-monorepo#1684:

• PR-A — tutorial (nevermined-io/tutorials#51, draft, pending smoke-test against sandbox)
• PR-B (this) — docs Use Cases section
• PR-C — multi-agent validation findings appended to agent-discoverability.mdx

Changes

File	Change
docs.json	New "Use Cases" group at top of the Guides tab (above Learn); two redirects for /docs/use-cases and /docs/use-cases/
docs/use-cases/overview.mdx	New. Landing page, cards-list of all use cases (currently one).
docs/use-cases/monetize-paid-apis-for-agents.mdx	New. Thin hub page for the Exa-style vend-the-key pattern. Strategic framing, decision guide (vend-the-key vs full metering), cross-links to solutions/api-providers.mdx, the tutorial, the case study, and Exa's live integration.
docs/use-cases/agent-discoverability.mdx	New, substantive content. The llms.txt + nevermined.md convention, fields to advertise, complete copy-paste templates (lifted from the tutorial), hosting guidance, a 3-step "test before agents see it" checklist, and a "Known agent rakes" placeholder that PR-C will populate.
docs/use-cases/case-study-exa.mdx	New. Short case study: what Exa shipped (one endpoint, no metering changes), why this is strategically interesting (monetizing services, not just agents), and what other providers should take away.

Out of scope

• Fixing the verify→provision→settle ordering on solutions/api-providers.mdx. The page's code samples (TS lines 78-80, Python lines 141-155, HTTP pseudo-code) show the unsafe ordering that PR-A's code review caught. Will be filed as a separate issue on nvm-monorepo after this merges, per @r-marques.
• Multi-agent validation. That's PR-C. The "Known agent rakes" section on agent-discoverability.mdx is a placeholder until then.

Test plan

• mintlify broken-links clean (verified locally)
• mintlify a11y clean on the new MDX files (site-wide colour-contrast issues are pre-existing, unrelated)
• All {{PLACEHOLDER}} strings inside MDX paragraphs are wrapped in backticks so MDX doesn't try to parse them as JSX expressions; placeholders inside code fences are literal
• Manual review of the four pages (someone other than me reads them and says they make sense)
• Verify the new section renders correctly in mintlify dev preview (left for reviewer)

Notes for reviewers

• The hub page (monetize-paid-apis-for-agents.mdx) is deliberately thin. If it feels like it's covering ground that solutions/api-providers.mdx already covers, that's a real concern to flag — happy to slim it further or restructure.
• The copy-paste template on agent-discoverability.mdx uses a four-backtick code fence to wrap a markdown template that itself contains three-backtick code fences. If that renders weirdly in your local preview, let me know.
• Draft until: (a) PR-A smoke-test confirms the templates actually work end-to-end, and (b) you've had a pass over the prose.

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