docs: add error message strings to relevant feature pages

[devin-ai-integration]

Summary

Adds "Common errors" sections to existing docs feature pages so users can paste an error string from fern check / fern generate --docs into docs search (or a search engine) and land on the right page. Per Devin's guidance in the Slack thread, errors live alongside the relevant feature rather than in a standalone troubleshooting page.

Errors were pulled from the fern CLI docs validator rules (packages/cli/yaml/docs-validator/src/rules/*) and the generate/generateDocsWorkspace command. Each accordion quotes the user-facing error string in its title and explains the fix.

Pages updated:

• seo/redirects.mdx — missing-redirects (moved/removed), circular-redirects (self-loop, chain)
• navigation/tabs.mdx — tab not declared, tab has both href + layout, tab missing any target
• navigation/frontmatter.mdx — Failed to parse frontmatter (inline <Info> next to the Special characters table)
• authentication/rbac.mdx — role used but not declared
• preview-publish/setting-up-your-domain.mdx — three Invalid URL ... variants for instances[].url
• preview-publish/preview-changes-locally.mdx — No docs.yml file found, broken link, invalid URL, Path ... does not exist
• preview-publish/publishing-your-docs.mdx — No token found ..., OpenAPI spec validation failure

Scope is intentionally "docs only, to start" — SDK and other error surfaces are not covered here.

Review & Testing Checklist for Human

• Spot-check a couple of error strings against what Fern actually prints. I lifted the wording from source rule files (e.g. valid-markdown-link, missing-redirects-logic, valid-instance-url, all-roles-must-be-declared, tab-with-href, filepaths-exist, valid-frontmatter, generateDocsWorkspace), but I didn't run the CLI against a broken fixture to confirm the exact casing/punctuation users see.
• For setting-up-your-domain.mdx, verify the listed valid domains (docs.buildwithfern.com, docs.dev.buildwithfern.com) match the full VALID_FERN_DOMAINS list in the validator. I used two examples; the real list may differ.
• Confirm any new cross-links resolve on the preview, especially /learn/cli-api-reference/cli-reference/commands#fern-generate and /learn/docs/writing-content/markdown-basics#link-format. Other anchors used (#fern-token) already appeared elsewhere in the repo.
• Sanity-check the accordion titles render correctly — a few contain quotes, parentheses, and an arrow character (→). If any look off in preview, the title attribute may need different quoting.

Test plan: open the preview URL, visit each updated page, expand each new accordion, and confirm content renders and internal links work.

Notes

• fern check on the branch surfaces the same 2 pre-existing SDK errors as main (CreateCodeRecordRequest / CreateDocumentRequest already declared) — unrelated to this PR.
• Vale isn't installed in this environment, so style checks weren't run locally. CI should cover it.
• No changelog entry added since this is content-only additions to existing pages, not a new feature.

Link to Devin session: https://app.devin.ai/sessions/7b48f89348e04d43bf5d6518e7f9939a
Requested by: @devalog

[devin-ai-integration]

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[github-actions]

📝 [vale] _{reported by reviewdog 🐶}
[FernStyles.Adverbs] Remove 'separately' if it's not important to the meaning of the statement.

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1776387241-docs-error-strings.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/authentication/features/rbac
• learn/docs/configuration/page-level-settings
• learn/docs/configuration/tabs
• learn/docs/preview-publish/preview-changes
• learn/docs/preview-publish/publishing-your-docs
• learn/docs/preview-publish/setting-up-your-domain
• learn/docs/seo/redirects