Remove bson v3 from driver docs

Currently the driver at https://docs.mongodb.com/ruby-driver/master/bson-tutorials/ has bson v3 and v4 tutorials. In practice the driver has been requiring bson 4 for at least the last couple of years, hence the v3 tutorial is not relevant to anyone looking at current driver documentation and we should probably get rid of it.

We should also ensure it is possible to get to the v3 tutorial through somewhere. I believe simply removing the v3 section from master would preserve it in all existing driver branches which should meet this requirement.