More fixes related to sticky marketing header

[colleenmcginnis]

Expands on the changes in #3126

Before

• When scrolling to a heading, the heading is covered by the marketing header.
• The table of contents on the left side is cut off at the bottom.

before.mov

After

• When scrolling to a heading, pin the heading just below the marketing header.
• The last item in the table of contents is fully visible.

after.mov

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[lcawl]

Thanks for the fix!