Documentation: CSP Headers page should note that a cache clear is required

Nobody has claimed this yet.

### Current Behavior

**Steps to reproduce the behavior**
1. Edit the config/app.yml file to change the Content Security Policy configuration
2. Restart php-fpm
3. Refresh the site in the browser. The changes to CSP have not taken effect.

### Expected Behavior

The new CSP takes effect.

### Possible Solution

https://www.accesstomemory.org/en/docs/2.10/admin-manual/security/csp-headers/#atom-s-csp-configuration

It currently says says "Updating these settings will require restarting [PHP-FPM](https://www.accesstomemory.org/en/docs/2.10/admin-manual/maintenance/troubleshooting/#troubleshooting-restart-php-fpm)." Instead, it should state "Updating these settings will require restarting [PHP-FPM](https://www.accesstomemory.org/en/docs/2.10/admin-manual/maintenance/troubleshooting/#troubleshooting-restart-php-fpm) and [clearing the cache](https://www.accesstomemory.org/en/docs/2.10/admin-manual/maintenance/clear-cache/#maintenance-clear-cache)."

Clearing the cache should also be mentioned in the next paragraph down, where it mentions restarting PHP-FPM again.

### Context and Notes

_No response_

### Version used

AtoM 2.10.1

### Operating System and version

Ubuntu 24.04

### Default installation culture

en

### PHP version

PHP 8.3

### Contact details

_No response_

Open the CSP Headers page linked in the issue and review the two paragraphs that currently mention restarting PHP-FPM. Update both to mention clearing the cache as well, using the provided cache-clearing link, then verify that the wording and links match the requested behavior.

Written by the indexing model from the issue text.