[WIP] Refactor docs

Fix #94.
i think it makes sense to have the rules reference separate, but then we should remove it from reference/configuration/headers.rst and reference to this part. 

another option would be to make this a "partial" and include it in all 3 places - still with some note that this is exactly the same for headers, tags and invalidation, as the user who uses all those features wants to understand that.