Document number of threads configuration depending on the machine available bandwidth

For instance, how many threads can we have when the machine available bandwidth is 100Mbps or 1Gbps. Based on what we talked in https://trac.torproject.org/projects/tor/wiki/org/meetings/2019BrusselsNetworkTeam/Notes/SBWSRoadmap#Questions

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information