docs: update header

[chenxsan]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update

To keep the top border of header consistent with other pages like Team/Blog which would take up the full width of viewport.

[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

1. Move .docs class to .docs-wrapper element

Before

After

Is there anything you'd like reviewers to focus on?

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[snitin315]

LGTM, thanks!

[kecrily]

LGTM

[mdjermanovic]

@chenxsan I had to revert this change before the latest release because it seems to break the design on mobile (please see #16451). Can you please open an issue describing the reason this change was made, so we could evaluate it and then perhaps try again to fix the problem? :)