Document excludeChangelog query parameter for llms.txt

[fern-api]

Fern-hosted docs sites serve a /llms.txt index that AI agents use to discover a site's content. On sites with long release histories, changelog entries can make up most of that index while adding little value for agents. /llms.txt now accepts an excludeChangelog=true query parameter that omits changelog sections (the changelog overview link and the nested changelog llms.txt index), so customers can hand agents a smaller, more relevant index. It composes with the existing lang and excludeSpec parameters.

Implements docs for: feat(docs): add excludeChangelog query parameter to llms.txt (fern-api/fern-platform#14849)

Pages changed

• fern/products/docs/pages/ai/llms-txt/customize-llms-txt.mdx: adds excludeChangelog to the "Filter endpoint output" intro, examples, and a new <ParamField>.
• fern/products/docs/pages/ai/llms-txt/analytics-integration.mdx: mentions excludeChangelog=true alongside lang and excludeSpec in the SDK button guidance.
• fern/products/docs/pages/changelog/2026-09-18.mdx: new Docs changelog entry (tag ai) linking to the filter section.

Validated with pre-commit (Vale) on changed files and fern check --warnings (0 errors).

[github-actions]

🌿 Preview your docs: https://fern-preview-2026-09-16-fern-ai-1fb7a4-71caaf36.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/llms-txt-analytics
• learn/docs/ai-features/customize-llm-output
• learn/docs/changelog/2026/9/18