docs: add dev note about auth strategy hot-reload requirement

[ilhamsj]

What?

Added a documentation note informing developers that custom authentication strategies require a full server restart and are not hot-reloaded during development.

Why?

During development, developers may expect auth strategies to hot-reload like other code changes in Payload. When changes don't take effect immediately, it leads to confusion and wasted debugging time trying to figure out why their strategy modifications aren't working.

This small documentation addition can save developers significant time and frustration.

How?

• Added an informational <Banner> component in docs/authentication/custom-strategies.mdx
• Placed it immediately after the existing warning banner at the top of the page
• Includes a clear note about the restart requirement and suggests using nodemon for auto-restart

Before:
Only had the warning banner about this being an advanced feature.

After:
Now includes an additional info banner that explicitly states auth strategies require restart and are not hot-reloaded.

Fixes: N/A (proactive documentation improvement)