Add a guide about ZNC

[brunnre8]

Closes #149.

First draft of how a znc guide could look like.
May you please have a look and tell me what needs to be improved?

[brunnre8]

Alternative layout as per @Zarthus suggestion

## What is ZNC?

[...]

## What does it do that The Lounge doesn't?

[...]

## I'm convinced, how do I set it up?

[install]

### Connecting to ZNC using The Lounge

[connecting]

[astorije]

Thank you so much for your help with this, @brunnre8! As you can see, your guide is now available at https://thelounge.chat/docs/guides
Hope you requested your stickers, you truly deserve them :)