Add difficulty bomb info to frontier-thawing

[zjiekai]

Description

There seeems to be this misconception that eip-2 introduced the difficulty bomb.

the client will calculate the difficulty based on a fake block number suggesting the client that the difficulty bomb is adjusting around 6 million blocks later than previously specified with the Homestead fork.

ethereum/EIPs#1234

Due to the "difficulty bomb" (also known as the "ice age"), introduced in EIP #2, an artificial exponential increase in difficulty until chain freeze,

https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1227.md

When looking into the homestead fork (specified in eip-2), it seems that the difficulty change has nothing to do with the difficulty bomb. The difficulty bomb was introduced in the frontier-thawing fork.

starting from block 200,000 (very roughly 17 days from now), the difficulty will undergo an exponential increase which will only become noticeable in about a year.

https://blog.ethereum.org/2015/08/04/ethereum-protocol-update-1/

A hard fork of Ethereum at block 200,000 to introduce an exponential difficulty increase (aka difficulty bomb), motivating a transition to proof-of-stake.

https://ethereum.org/en/glossary/#ice-age

This information is already in the eth.org site at the ice-age glossary. It's nice to have this at the history page.

There are 2 blog posts about the frontier-thawing fork. This PR adds the link to the other one.

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[minimalsm]

Thanks @zjiekai 😀