docs: document redirect pattern syntax, including regex parameters

[devin-ai-integration]

Summary

The redirects page only documented :slug and :slug*, but redirect matching passes the source straight to path-to-regexp 6.3.0, so regex-constrained parameters (:slug(.*), :version(\d+)) and unnamed groups have worked since fern-platform#1127 (Jul 2024) without ever being written down. Customers were left guessing whether mid-segment patterns were supported or incidental.

Adds a ## Pattern syntax section with the five usable forms, plus the two behaviors that aren't discoverable from the syntax itself:

• The regex must be repeated in destination. source: "/plants-:slug(.*)" with destination: "/garden/:slug" fails to compile (a bare :slug can't hold a value containing /) and safeCompile falls back to emitting the destination verbatim — the user gets a redirect to the literal string /garden/:slug.
• ? is unsupported. matchPath strips each source at the first ? to drop search params, so /old/:slug? is matched as /old/:slug and the optional modifier silently disappears.

Every claim was verified against the pinned path-to-regexp@6.3.0 by running the documented examples through getRedirectForPath:

✓ mid-segment regex param      /plants-:slug(.*)  → /garden-monstera/care   (/plants-:slug does not match)
✓ constrained param            /v:version(\d+)/…  → version = "2"
✓ unnamed group replays as :0  /plants/(\d+)      → /garden/42
✓ destination must repeat regex                   → /garden/:slug (literal)
✓ optional modifier stripped   /plants/:slug?     → no match for /plants

The zh translation is updated in the same shape.

Link to Devin session: https://app.devin.ai/sessions/ad30ef5780fe4114bad5fa79b1d195af

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1786111850-document-redirect-pattern-syntax.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/seo/redirects