Make email address optional for Let's Encrypt SSL certificate issuance

Is your feature request related to a problem? Please describe.
Currently, when obtaining an SSL certificate from Let's Encrypt through Nginx Proxy Manager, users must provide an email address. However, Let's Encrypt does not require an email for certificate issuance. This requirement can frustrate users who wish to protect their privacy or simply do not need email notifications regarding their certificates.

Describe the solution you'd like
I propose updating the certificate request workflow to allow the email field to be optional. When the field is left empty, the system should:

• Proceed with certificate issuance without error.
• Clearly indicate that no certificate-related notifications (such as renewal warnings or error messages) will be sent due to the absence of an email address.
• Update documentation to reflect this change and to inform users of the implications.

Describe alternatives you've considered
One alternative is to require a placeholder or dummy email address, but this does not respect user privacy or align with Let's Encrypt's policy. Another option might be to allow an initial email entry with the ability to opt out later; however, this still forces users to share an email at setup. Allowing users to leave the email field blank entirely is a more straightforward and privacy-forward solution.

Additional context
Making the email optional will improve the user experience and align Nginx Proxy Manager more closely with Let's Encrypt's non-mandatory email requirement. This enhancement would especially benefit users who prefer not to receive notifications or share personal data, resulting in cleaner and more flexible certificate management.

The issue names no files, tests, or entry points; start by tracing the certificate request workflow and its Let's Encrypt integration, including where the email is required. Done means an empty email completes issuance, the UI explains that notifications are unavailable, and documentation states the implications.