fix: desktop/mobile ToC overflow scrolling

[wackerow]

Description

• Adds max height to desktop and mobile table-of-contents (ToC) components
• Adds 1rem fade to top of "docs" ToC as indication to user when ToC is not at it's top position (invisible at top due to 1rem top padding)
• Adds 4rem bottom padding for when ToC at end scroll position to avoid overlap with feedback widget

Related Issue

• Fixes scroll issues (Desktop must scroll whole page to bottom before able to get to end of long ToC; Mobile unable to scroll long ToC dropdown)
• Fixes FeedbackWidget overlapping with last items (with long list of items)

Preview link

https://deploy-preview-16574--ethereumorg.netlify.app/whitepaper

cc: @konopkja

[netlify]

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