Update ActivityTemplate docs

[iarovikov]

This document
https://github.com/Fr8org/Fr8Core/blob/master/Docs/ForDevelopers/Objects/ActivityTemplates.md

lack of some very important point - activity template id should never change over the time. This is critical to mention.

It appears that "categories" is mandatory. If a developer skips setting this property then related activity template will not be displayed in the UI

Also this doc has broken formatting that should be fixed.