Document APIs for service account volume projection

[tengqm]

This is a...

• [*] Feature Request
• Bug Report

Problem:
The APIs for service account volume projection and the related feature gate needs a doc.
xref: kubernetes/kubernetes#63819

Proposed Solution:
Document it.

Page to Update:
May be a new page similar to an existing one: https://github.com/kubernetes/website/blob/master/content/en/docs/tasks/inject-data-application/environment-variable-expose-pod-information.md

v1.11

[tengqm]

/assign