docs: add dev note about auth strategy hot-reload requirement (#14122)

### 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)