docs: make it more clear that one can self host

[chapmanjacobd]

I've used vopono several times but somehow I've missed that it can be used with your own OpenVPN server

I think putting something like a "Self" provider in the table would make it a lot more clear. Open to edits on the specific language but I definitely think it should be in the table somewhere because some people only look at tables

[jamesmcm]

Thanks!