feat: peer discovery and routing section

[salmad3]

Docs this PR introduces that were already approved

• https://github.com/libp2p/docs/pull/260
• https://github.com/libp2p/docs/pull/290
• https://github.com/libp2p/docs/pull/259

Context

• Creates a new section for peer discovery and routing.
• Transforms the initial intent with the discovery section to capture "peer discovery and routing"
• resolves #250, #13

[salmad3]

Link fixes and potential conflicts will be addressed in this PR once all associated material has been added.

[mxinden]

@p-shahi is this still planned to merge eventually? If so, I would push a couple of changes to the mdns document.

[p-shahi]

@mxinden please push your changes. I will merge by end of week