fix(docs): broken /apps/introduction/overview link in verify-social-accounts guide

## Bug

`docs/base-account/guides/verify-social-accounts.mdx` line 435 links to `/apps/introduction/overview` which returns 404 on the live site.

**Steps to reproduce:**
1. Visit https://docs.base.org/base-account/guides/verify-social-accounts
2. Scroll to the end — click 'Apps overview' link
3. Gets 404

**Root cause:**
`docs/apps/introduction/overview.mdx` has `hidden: true` in frontmatter so it is not publicly accessible.

**Fix:**
Replace `/apps/introduction/overview` with `/apps` which returns 200.

**Affected file:**
`docs/base-account/guides/verify-social-accounts.mdx` line 435