openvpn guide pages in mdbook

[cachebag]

part of #288.

add openvpn documentation to the mdbook:

• guide/vpn-openvpn.md — openvpn setup, auth types, builder usage
• update guide/vpn.md to cover both wireguard and openvpn
• update guide/vpn-management.md — openvpn appears in list/info/forget
• add example: examples/openvpn-client.md
• add example: examples/ovpn-import.md
• update SUMMARY.md with new pages
• update api reference pages with new types