Write fully featured MAC +ARP + IP example

[rowanG077]

Feature description
We would like to have a few example stacks for users of our core. This one is for a fully featured MAC + ARP + IP example. The module should live in Clash.Cores.Ethernet.Examples. For the PHY use a dummy with clear documentation that a user should use one for their specific protocol (RGMII. MII, etc)

Expected behaviour
A clearly documented example. This should be in the form of a (large )haddock comment including a doctest.

Expected steps
Implement example.

Additional Context
See https://github.com/clash-lang/clash-compiler/blob/948ad7d486350ef99c41ee4b7c8cec2857fde49e/clash-cores/src/Clash/Cores/Crc.hs for an example how to write haddock examples including doctest.