[WIP] Refactor docs

Fix #94.
configuration/rules.rst is this file i am commenting on. if i get it correctly, this just lists what you can do for `match` in rule sets. this makes sense, but it looks like you copied this from configuration/headers.rst and did not remove it there. i think the best would be to make this a "partial" and adapt it so it can be included in all reference sections that talk about rules, to explain the match concept and options.

an example is https://github.com/symfony-cmf/symfony-cmf-docs/tree/master/reference/configuration/partials and `.. include:: partials/persistence_phpcr_enabled.rst.inc`