Rename to eth2-networks

[arnetheduck]

We have two repos keeping track of known eth2 networks - this one and https://github.com/eth2-clients/eth2-mainnet - life can be made easier by simply renaming this one to eth2-networks and deprecating eth2-mainnet - conveniently, the mainnet data is already in this repo and more recent than the eth2-mainnet data (eth-clients/eth2-mainnet#5)

[arnetheduck]

From a practical pov, the transition is simplified by github putting in place a redirect from the old name, so existing links won't break.

[ajsutton]

Sounds good to me.

[protolambda]

Sounds good

[terencechain]

sounds good