Fix section numbering (try Markdown automatic numbering)

[cdanger]

Currently, section numbers are edited manually, which is error-prone. For instance, while making changes for XACML 4.0, a few sections have disappeared: 3.3.1.3, 5.8, 5.9, 5.10, 5.20, 7.6, 7.13...

So numbering should be fixed at some point. A better approach on the long-term is to use automatic numbering with Markdown, so that we don't have to worry about that anymore. Not sure this works for section numbers mixing letters and numbers like Appendix sections. To be checked.

Downside: Github markdown does not support (automatically) numbered sections, so the section numbers would be visible only in the pandoc HTML/PDF output (using --number-sections=true option).