[Docs] improve NAT and manual peering documentation

[claudiolor]

Description

This PR improves documentation about manual peering and the way to configure the network when there is a NAT in the topology

[adamjensenbot]

Hi @claudiolor. Thanks for your PR!

I am @adamjensenbot.
You can interact with me issuing a slash command in the first line of a comment.
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Make sure this PR appears in the liqo changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
• kind/feature: 🚀 New Feature
• kind/bug: 🐛 Bug Fix
• kind/cleanup: 🧹 Code Refactoring
• kind/docs: 📝 Documentation

[frisso]

Two comments on the picture.

1. Typo: "gatway" instead of "gateway" :-)
2. To me, one of the biggest advantage of the manual Liqo peering is that for the network setup we don't care who is the provider, and who is the consumer. I think this should be stated clearly in the documentation, ans possibly reflected in the picture, where clusters could be named "cluster1" and "cluster2" instead of "consumer/provider".

Finally, did you try if SVG images (instead of PNG) work?
Apparently they are supported now: https://stackoverflow.com/questions/13808020/include-an-svg-hosted-on-github-in-markdown