fix(docs): remove .mdx extension from signInWithEthereum link in wallet_connect reference

[ygd58]

Bug

docs/base-account/reference/core/provider-rpc-methods/wallet_connect.mdx line 193 links to /base-account/reference/core/capabilities/signInWithEthereum.mdx with a .mdx extension which returns 404.

Steps to reproduce:

1. Visit https://docs.base.org/base-account/reference/core/provider-rpc-methods/wallet_connect
2. Click the signInWithEthereum link
3. Gets 404

Root cause:
Link incorrectly includes .mdx extension:

• ❌ /base-account/reference/core/capabilities/signInWithEthereum.mdx → 404
• ✅ /base-account/reference/core/capabilities/signInWithEthereum → 200

Affected file:
docs/base-account/reference/core/provider-rpc-methods/wallet_connect.mdx line 193