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:

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

steps to complete the merge successfully:

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