Merge pull request #2352 from lemoer/pr_site_vpn_mtu_in_provider

contrib/ci/minimal-site/site.conf

docs/multidomain-site-example/site.conf

docs/site-example/site.conf

docs/user/site.rst

package/gluon-mesh-vpn-core/check_site.lua

...gluon-mesh-vpn-core/luasrc/lib/gluon/upgrade/500-mesh-vpn

package/gluon-mesh-vpn-fastd/check_site.lua

...esh-vpn-fastd/luasrc/lib/gluon/upgrade/400-mesh-vpn-fastd

...astd/luasrc/usr/lib/lua/gluon/mesh-vpn/provider/fastd.lua

package/gluon-mesh-vpn-tunneldigger/check_site.lua

...asrc/usr/lib/lua/gluon/mesh-vpn/provider/tunneldigger.lua

package/gluon-mesh-vpn-wireguard/check_site.lua

.../luasrc/usr/lib/lua/gluon/mesh-vpn/provider/wireguard.lua