Provide information on available pinning services for FAQ

[konobi]

Currently documentation refers to the availability of pinning services for dat.

It's unclear as to where those services exist, which projects (either local or 3rd party) provide the functionality or what's entailed in running/using those services.

It would be good to do a small fact-finding mission to get an up-to-date understanding of the current state of pinning and make decisions on whether to document it or to remove "pinning" from current documents until it's better understood and supported.