Improve documentation on OCSP stapling

[tdelmas]

The "integration guide" https://letsencrypt.org/fr/docs/integration-guide/#implement-ocsp-stapling explains why it's important:

• Privacy
• Speed
• Costs for Let's Encrypt

But doesn't explain how to do it. Maybe a dedicated page could explain it, for most common servers?

Also:

• In the advantages, "avoid downtime in case of revocation" could be added
• The sentence about cost could be updated if new data is available (https://letsencrypt.org/2018/12/31/looking-forward-to-2019.html says 5.5 billion requests a day)