Add tutorials/blog posts about securing a Prometheus setup

[fabxc]

Those only have to cover the most basic scenarios to secure a Prometheus setup so people have a base for further customization.

[grobie]

Should we add a "Howto" section?

[fabxc]

Good idea.

Many blog posts will be how-tos. So this section would probably be a mix of direct content and references to those posts so one doesn't have to dig around in the blog archive (which we don't have right now anyway).