docs: document Fern's two MCP servers and make agent setup human-visible

[devin-ai-integration]

Summary

Fern now hosts two MCP servers — the public documentation server and the authenticated organization server (https://fai.buildwithfern.com/mcp) — so this PR gives them a canonical page and reworks the quickstart mentions to point at it instead of introducing each server inline. It also absorbs #6575 (closed), which documented the fern mcp install CLI command for the same feature.

Implements docs for: feat(cli): add fern mcp install to connect coding agents using the login token (fern-api/fern#17336)

New page /learn/docs/ai-features/fern-mcp-servers. A table of both servers, per-client setup (Claude Code, Codex, Cursor, Copilot) in <CodeBlocks>, what the org server exposes (sites and config, deployments, analytics, reader signals, members), and the two operational facts agents get wrong: the OAuth login needs a human at the browser, and a server registered mid-session isn't callable until the next session. Capabilities and read-only scope were verified against fai/agent/mcp_server.py and the tool registry (docs-editing tools carry exclude_access_from={"mcp"}), including the org-scoped /organizations/{org}/mcp URL that multi-org accounts need.

fern mcp install is the primary setup path for the org server (folded in from #6575). The command reuses the token fern login already stored, so it replaces the manual per-client OAuth registration everywhere the org server is set up — the canonical page leads with it and keeps hand-registration as the alternative, and the quickstart's post-publish section is now fern mcp install rather than a claude mcp add one-liner. The CLI reference gets a fern mcp install accordion covering the fern login prerequisite, the config file written per client, the FERN_TOKEN rejection, the non-refreshing token, and the --client / --org flags; conceptual detail lives on the canonical page, which it links to.

"Set up your coding agent" is no longer llms-only in both the Docs quickstart and the SDK init-fern-folder snippet. Humans now see ~3 lines plus the Claude Code commands wrapped in <llms-ignore>, framed as "this is what your agent is about to do"; the full per-client instructions stay in <llms-only> unchanged. This also fixes the previous ordering problem where the public server first appeared, unexplained, in the post-publish org section.

Cross-links from ai/overview (card group, replacing the skills-only section), ai/mcp-server (distinguishing your site's generated server from Fern's), and agent-skills.

Also merges main to clear the conflict, which was an add/add on the 2026-08-06 changelog file — both entries are kept, and the MCP entry now mentions fern mcp install. Section headings on the new page dropped their Public: / Private: prefixes to satisfy Microsoft.HeadingColons.

Link to Devin session: https://app.devin.ai/sessions/f3cc1ad7109d48939b43d6ee8c30f180

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1786035479-authenticated-mcp-quickstart.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-api-reference/cli-reference/commands
• learn/docs/ai-features/agent-skills
• learn/docs/ai-features/fern-mcp-servers
• learn/docs/ai-features/host-skills
• learn/docs/ai-features/mcp-server
• learn/docs/ai-features/overview
• learn/docs/changelog/2026/8/6
• learn/docs/getting-started/quickstart

[devin-ai-integration]

Superseded by #6672, which carries all of this PR's content combined with #6624. Closing in favor of that PR.