content: Cover web3signer in validator docs.

[zengzengzenghuy]

What

web3signer is a remote signing tool developed by Consensys. Validator client may outsource signing task to web3signer.
Using web3signer is only recommended for advanced node runner.

Currently, the content is covered in Dappnode section. Should cover this in a dedicated section to explain how to use web3signer.

How

• Install and Setup web3signer
• Configure web3signer on validator clients (Lighthouse, Teku, etc)

Ref

1. https://docs.web3signer.consensys.net/
2. Lighthouse: https://lighthouse-book.sigmaprime.io/validator-web3signer.html
3. Prysm: https://docs.prylabs.network/docs/wallet/web3signer
4. github