Fix legacy Agents SDK and SWML redirects to equivalent pages

[Devon-White]

Refs #526 (epic #559).

Legacy Agents SDK links currently collapse to the Server SDK overview, and old SWML paths either take an extra hop or return 404. This change maps known old paths directly to equivalent existing pages.

• Cover the original Agents SDK Python URLs and legacy short forms, including CLI tools, ContextBuilder, configuration, DataMap, and result actions.
• Send the old SWML schema page to the existing SWML guide, which covers document structure and validation.
• Remove the proposed overview fallbacks for best practices, patterns, migration, changelog, and the two examples pages. No equivalent current page was found for these six pages, so their old paths remain not found instead of redirecting to unrelated content. Exact entries also prevent unknown paths from redirecting to nonexistent guides.
• Map legacy SWML method names and merged property pages to the current calling reference, including underscore/hyphen aliases and SWAIG defaults.

Only fern/docs.yml changes. No documentation content is added, moved, restored, or rewritten.

Validation on commit d38fd976ca: both CI jobs passed, including the authenticated Fern published-site comparison and markdown checks. The deployed preview passed all 307 URL probes: 295 redirects reached the expected page in one hop with HTTP 200, and 12 no-successor paths returned HTTP 404 without redirecting. Five initial network timeouts passed on retry. The local audit also checked current page slugs, duplicate sources, source collisions, and additional redirect hops.

The developer.signalwire.com side is a separate docs-legacy redirect map patch. Fragment preservation and a permanent redirect-integrity CI gate remain outside this PR.

[github-actions]

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https://signalwire-preview-docs-663.docs.buildwithfern.com/docs

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[Devon-White]

Verified against the preview deploy (signalwire-preview-docs-663). Every legacy path below returns a single 308 to the exact page:

Legacy path	Lands on
/docs/agents-sdk/security	/docs/server-sdks/guides/security
/docs/agents-sdk/deployment/production	/docs/server-sdks/guides/production
/docs/agents-sdk/deployment/serverless	/docs/server-sdks/guides/serverless
/docs/agents-sdk/python/guides/voice-language	/docs/server-sdks/guides/voice-language
/docs/agents-sdk/python/guides/security	/docs/server-sdks/guides/security
/docs/agents-sdk/python/reference/agent-base	/docs/server-sdks/reference/python/agents/agent-base
/docs/agents-sdk/python/reference/cli-swaig-test	/docs/server-sdks/reference/python/agents/cli/swaig-test
/docs/agents-sdk/python/reference/contexts	/docs/server-sdks/reference/python/agents/context-builder
/docs/agents-sdk	/docs/server-sdks
/docs/server-sdk/guides/security	/docs/server-sdks/guides/security
/docs/swml/methods/ai/languages	/docs/swml/reference/calling/ai/languages
/docs/swml/reference/calling/ai/swaig/defaults	/docs/swml/reference/calling/ai/swaig
/docs/swml/reference/ai/swaig/defaults	/docs/swml/reference/calling/ai/swaig
/docs/swml/reference/ai/swaig/functions	/docs/swml/reference/calling/ai/swaig/functions

CI (run, preview) green, including the missing-redirects check that could not run locally.

[Devon-White]

Redirect-only audit corrections are pushed to PR #663 at d38fd976ca. No documentation content was added, moved, restored, or rewritten.

• Known Agents SDK guide/reference URLs and short forms now map directly to existing equivalents, including the renamed DataMap/result-actions paths and SWML schema → the existing SWML guide.
• Removed misleading overview fallbacks for six pages without equivalents: best practices, patterns, migration, changelog, examples by complexity, and examples by feature. Their twelve original/short URL forms return 404.
• Legacy SWML method and property paths go directly to the current references, including renamed verbs and merged SWAIG fields.

Both CI jobs passed. All 307 deployed-preview probes passed: 295 one-hop redirects ending at the exact expected page (HTTP 200), plus 12 direct 404s. Five initial timeouts passed on retry.

The separate docs-legacy map patch is prepared but not pushed. Its 330 corrected redirect entries were tested with the repository's actual nginx configuration; all 114 distinct destinations returned production HTTP 200 without another redirect. Twelve exact map exclusions return real 404s instead of falling through to the docs-root catch-all. A small companion test-script patch handles those intentional exclusions. Fragment preservation and a permanent redirect-integrity CI gate remain outside this change.