docs: add /pricing as Ask Fern datasource

[devin-ai-integration]

Summary

Adds https://buildwithfern.com/pricing as an ai-search.datasources entry in fern/docs.yml so Ask Fern crawls and indexes the marketing-site pricing page alongside /learn content.

Context: asking Ask Fern "pricing" today returns "I wasn't able to find specific pricing information…" followed by a link to /pricing. The first paragraph is not a configurable default — it's the LLM responding honestly because the pricing page lives on the marketing site (not /learn) and was never in the Ask Fern index. After this change, pricing questions will retrieve the actual pricing page and cite it directly.

Alternatives considered:

• Websites API with path_filter — same outcome, more moving parts. Using datasources is declarative and lives with the rest of the docs config.
• Guidance API entry — useful for sensitive/canned responses, but the public pricing page has enough detail that letting the LLM summarize it with a citation is better than a hand-written redirect.

Review & Testing Checklist for Human

• Confirm https://buildwithfern.com/pricing is the correct canonical URL to index (vs. a more specific subpath) and that it's OK for Ask Fern to cite it on buildwithfern.com/learn/*
• After merge + reindex, run the same query ("pricing", "how much does Fern cost", "what plans does Fern have") against Ask Fern and verify it cites /pricing instead of the fallback "not found" response
• Sanity-check that no other marketing pages (/customers, /careers) should be added at the same time

Notes

Schema reference: datasources is defined on AiChatConfig in packages/cli/configuration/src/docs-yml/DocsYmlSchemas.ts in fern-api/fern, and documented at https://buildwithfern.com/learn/docs/ai-features/ask-fern/content-sources.

Link to Devin session: https://app.devin.ai/sessions/1073da69838b4d9da504b641c3da3506
Requested by: @devalog

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