docs: improve llms.txt routing

[Devon-White]

Summary

• restructure the LLM-only documentation homepage as a customer-facing routing guide organized by build path and popular feature area
• update the homepage frontmatter description used for Open Graph metadata and the root Markdown export, replacing legacy “Realtime SDKs” wording with current product language
• replace overlapping comparison tables with scannable headings and grounded links to public landing, getting-started, and reference pages
• recommend Call Flow Builder only for explicitly low-code or no-code use cases and keep the Compatibility API scoped to Twilio migration
• tell agents to append .md or request Accept: text/markdown for clean page exports and use the documentation indexes listed by Fern in the root llms.txt
• keep concise navigation guidance consistent across the global page directive and visible LLM hint, with additional root-specific context in the root llms.txt
• preserve the targeted 26-case, 120-route LLM export checker and update its homepage assertions for the new structure
• make the corpus export audit strip any leading Fern page directive before checking for empty bodies or soft 404s, independent of the directive wording

Validation

• yarn fern-md-check passes
• node --test "scripts/**/*.test.js" passes all 41 tests
• git diff --check passes
• verified that Accept: text/markdown returns clean Markdown exports
• verified all 69 unique internal documentation links in the root LLM routing block resolve to public pages with matching page titles
• verified the updated homepage and root-index manifest assertions against the current <llms-only> content without an expected-output fixture

Validation limitation

• yarn fern-check reaches Fern's remote missing-redirects check but cannot complete because the FDR endpoint returns HTTP 403, including with external network access

[github-actions]

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https://signalwire-preview-docs-657.docs.buildwithfern.com/docs

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[hey-august]

Is this line necessary? This section sets off my "overprompting" alarms

[Devon-White]

Yeah I added that cause I had done a few personal test on both claude and codex where i fed them a doc where i didnt start from llms.txt. They both tend to not do a follow up fetch of the product llms.txt, unless asked specifically about a certain product.

Consider how Fern organizes their llms.txt. It does not contain all links for all docs. So unless it knows it needs to fetch a product llms.txt to get the rest of the links, he tends to just start googling instead.

I would recommend keeping all, except maybe that last line.