Title: Doc update: "life of a request"

Description:

• Reading the STYLE doc https://github.com/envoyproxy/envoy/blob/master/STYLE.md
• Has a link to "Life of a Request" which is 404
• Should go to https://github.com/envoyproxy/envoy/blob/master/docs/root/intro/life_of_a_request.rst
• Also that fie has broken images, so the links there need to be updated.