[CI] Optimize docs workflow; pip install -r requirements-docs.txt

[jbampton]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• No this is a CI update. The PR name follows the format [CI] my subject

What changes were proposed in this PR?

Standardize docs install process to use a requirements file.

How was this patch tested?

Did this PR include necessary documentation updates?

• No, this PR does not affect any public API so no need to change the documentation.