Add Dango Testnet How To Guide to docs

[sodofi]

On the home page of the docs under the build with minipay and before the learn how to build with Celo include a

'CEL2 Dango Testnet' [Header 1] with a Learn More link.

The Learn More link can connect to the Cel2 tab section. Edit index.md on Cel2 overview page to include:

Celo is transitioning from being an independent EVM-compatible Layer 1 blockchain to an Ethereum Layer 2. Learn more about the CEL2 announcement here.

Dango Testnet is the first Cel2 public test network for developers building on Cel2.

Also ensure that we trigger reindex so if someone searches for Dango they can find this page