fix: redirect /wallet-app/* catch-all to /build-on-base/overview

[forumevi]

What changed? Why?

Changed the /wallet-app/:slug* redirect destination in docs/docs.json
from /base-app/:slug* to /build-on-base/overview.

The previous destination forwarded to a path with no catch-all rule,
so any /wallet-app/<unknown> URL landed on a 404. Every other
/wallet-app/* rule already points to /build-on-base/overview, so
this makes the catch-all consistent.

Fixes #1992

Notes to reviewers

One-line change in the redirects array. No other rules touched.

How has it been tested?

Before: https://docs.base.org/wallet-app/example-page → 404
After the change is deployed: should redirect to /build-on-base/overview

Screenshots

N/A (no user-facing changes)

[cb-heimdall]

🟡 Heimdall Review Status

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[forumevi]

The linked issue #1992 documents the 404 repro. Let me know if you'd like
me to add a /base-app/:slug* catch-all as an alternative fix.