vpn/openvpn: Improve SSL VPN Client guide

[Monviech]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• I have searched the existing issues and I am convinced that mine is new.

Is your feature request related to a problem? Please describe.

Update SSL VPN Guide for OpenVPN Instances with a more easy to follow step by step approach. (Like the existing IPsec Roadwarrior guide for Connections)

Add optional sections for integrating LDAP and LDAP + TOTP, based on the new usermanager of 25.1.