running.rst: split out the server/introducer text, so someone who only
care about running a client doesn't need to read about hostnames or
--port/--location.
servers.rst: more background text on ports and locations, make section
names less storage-centric
* replace sample IPv4/IPv6 addresses with reserved ones from RFC-6890
* remove initial blank line: prevents github from rendering the .rst
* emphasize --hostname, then have --port/--location as a special-case
* list --port first (describe it "from the inside out"), then --location
* explain difference between --port and --location
* in endpoint strings, put interface= at end, to emphasize port
* add servers.rst to index.rst so it'll show up on readthedocs
* don't mention "partial-cone NAT": that's only relevant if/when we get
real ICE-style NAT-hole-punching
This includes configuring servers to use IPv4, IPv6, IPv6 with
port forwarding firewall and suggesting the use of i2p/tor if
NAT penetration is needed: provided links to configuration and
anonymity-configuration
