Write documentation for Configuration component

[RCheesley]

We need to bring over documentation relating to the Configuration component, update it to incorporate new additions and code samples, and convert it to RST format.

[dennisameling]

Some parts are already ported over: https://mautic-developer.readthedocs.io/en/latest/plugins/config.html

Only this still needs to be ported: https://developer.mautic.org/#custom-config-params

[mautibot]

This issue has been mentioned on Mautic Forums. There might be relevant details there:

https://forum.mautic.org/t/post-thanksgiving-boost-contribute-to-mautic-this-open-source-friday/30038/1