Advanced mod guide

[bitonality]

Clarifies #526
Fixes #525
Addresses some of the requested documentation in #513

[Buckminsterfullerene02]

@bitonality we are using mdbook for docs which means that;

• the new github markdown features like important, tip, caution etc. are not supported by it (as far as I know, I don't recall seeing it in their docs, but they do move fast so maybe they have it now). Otherwise I would've been using them a lot more before...
• when adding new files, you also need to update the page tree in SUMMARY.md. it's pretty self explanatory how that works once you see it

For either of the above you can manually merge docs-export and docs folders in the repo and download mdbook.exe to put in there so you can run a local copy to see how your changes look in it.