Improve docs for config.py

[alexfikl]

Adding configuration through config.py is very powerful, but we don't really document what can or should be done in there. It would be useful to expand that section in the documentation. Some things that could be documented

• Import order: config.py is loaded after the static configuration file.
• Usage of register_default_settings: the static config file takes precedence.
• Various dynamic features: add libraries dynamically, register papis doctor checks, etc.

xref: suggested by @octvs in #855 (comment)