Update Environment Lighting doc

[ValGrimm-U3D]

The Static Lighting Volumetric Clouds option is missing from the Environment Lighting page. In this PR, I add information about that option. I also added a note about the impact of the dominant directional light on the scene based on a tooltip Julien Ignace added to the UI in #6059. Based on official docs guidance regarding maintainability (https://unity-docs.gitbook.io/style-guide/format/images/screenshots), I have also removed the screenshot of the menu.

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Checklist for PR maker

• [ tbd as I am not sure if any backports are needed, so not yet] Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
• [ n/a] Have you updated the changelog? Each package has a CHANGELOG.md file.
• [ n/a] Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.
• [ n/a] Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

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Purpose of this PR

Why is this PR needed, what hard problem is it solving/fixing?
Updating the Environment Lighting page.

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Testing status

Describe what manual/automated tests were performed for this PR
n/a

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Comments to reviewers

Notes for the reviewers you have assigned.
I do not know which versions I will need to backport this to, if any, because I am not sure when Static Lighting Volumetric Clouds was introduced.

[github-actions]

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[github-actions]

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[anisunity]

LGTM