Use markdown syntax on agent and billing documentation

Hi,
I've read the contributing page, but fill a bug only for this is a bit overkill :-), it's just a little PR concerning markdown syntax:
* [x] Remove un-needed white-spaces in `doc/*`
* [x] Updated markdown syntax in `agent` and `billing` pages (previous format was hard to read on github)
* [x] Use linguist flavored markdown for other code/confs examples

Cheers,