Enhancement: Add current timeline of merge progress on the site with milestones crossed off as completed

[MiConnell]

Now that the merger devnet has successfully completed (but as vitalik pointed out this is only a test chain that's post-merge from genesis, not a full test of the actual transition) it would be great to use ethmerge as a living document of the progress of the merge outside of the faq.

Something like:

there are three major phases involved:

• [x] Phase 0: launch the beacon chain
• [ ] Phase 1: launch shard chains
• [ ] Phase 2: complete the merge 🎉

  steps to complete the merge successfully:

• [x] rough devnet
• [ ] longer new devnets
• [ ] public testnet
• [ ] months long testnet replicating the live network with fake eth
• [ ] merger (RIP miners)