docs: add global middleware section to middlewares page (fixes #4609)

[tech00exploere]

Description

The /docs/v4/middlewares/ page only showed middleware examples for the main namespace (/),
with no mention of how to apply middleware globally across all namespaces.

This was confusing because the existing how-to guide (/how-to/register-a-global-middleware)
already covered this topic but was not discoverable from the main middleware docs page.

Changes

• Added a new "Applying middleware to all namespaces" section at the bottom of the middlewares page
• Shows how to use the new_namespace event to register global middleware
• Includes a caution about namespace registration order
• Links to the existing how-to guide for additional patterns (manual registration, dynamic namespaces)

Related

Fixes socketio/socket.io#4609

[vercel]

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