<various>: Split "Firewalls" section into distinct sections.

[flexibeast]

At request of @the-maldridge, to facilitate expanding content.

@ericonr: Does this look okay for me to merge?

[flexibeast]

@ericonr: Hmm, no, i don't think it would be .... Whereabouts would you suggest?

[flexibeast]

@ericonr: i've added the contents of "Firewalls" to the end of network/index.md, and removed firewalls.md. Let me know if you think the content should go elsewhere on the page.

[Duncaen]

Is this really worth it, is there that much content planned?

[flexibeast]

@Duncaen: You'd have to ask @the-maldridge. :-)