Documentation improvement: Add "Purpose" section to Nostrum.Shard.Supervisor

[jchristgit]

All of our internal modules have a "Purpose" section with an admonition
mentioning that the module is internal and the documentation is only included
for people curious to look behind the curtain.

The Nostrum.Shard.Supervisor module and its documentation should include this
so users know what the shard supervisor is doing in their system, possibly with
a short piece of information on what to expect in a regular runtime
environment.