Add documentation about config

[robert-zaremba]

Summary

It seams we don't have any documentation about config files and major options.

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[robert-zaremba]

@clevinson , should we set it for 1.0 release? Or 0.40.x ?

[alexanderbez]

The template describes all the options. I think we just need to copy and lift into a markdown file maybe. Maybe incorporate it into running a node doc @amaurymartiny worked on.

[robert-zaremba]

If the template describe all options, then we don't need to copy them. We just need to make a nice section about config: describe what are the most important options and files. And then we will just say, for details please look at that files.

[robert-zaremba]

In that case this could be part of 0.40.x

[alexanderbez]

I think we should document all configs from that template in addition to what you've described.