feat(l1): add mainnet as preset network

[fmoletta]

Motivation
Adds mainnet bootnodes & genesis file so we can connect to mainnet by passing --network mainnet

Description

• add mainnet bootnodes & genesis file
• recognize mainnet as preset network
• update docs

Contributes to #72

Notes
Mainnet genesis file is quite large, it is copied from here

[github-actions]

Lines of code report

Total lines added: 13
Total lines removed: 0
Total lines changed: 13

Detailed view

+-----------------------------------+-------+------+
| File                              | Lines | Diff |
+-----------------------------------+-------+------+
| ethrex/cmd/ethrex/initializers.rs | 345   | +7   |
+-----------------------------------+-------+------+
| ethrex/cmd/ethrex/networks.rs     | 28    | +6   |
+-----------------------------------+-------+------+