[2021.2] Added a new document about Scene Templates

[JordanL8]

Purpose of this PR

Backport of #6009

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://unity-ci.cds.internal.unity3d.com/project/902/
Search for your PR branch using the search bar at the top, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

URP
/jobDefinition/.yamato%2Fall-urp.yml%23PR_URP_2021.2
With changes to URP packages, you should also run
/jobDefinition/.yamato%2Fall-lightmapping.yml%23PR_Lightmapping_2021.2

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[github-actions]

It appears that you made a non-draft PR!
Please convert your PR to draft (button on the right side of the page).
See the PR template for more information.
Thank you!

[Verasl]

LGTM