docs: Try to make people more successful with `cut_err`

[epage]

Looking back on #435, we need to

• raise the visibility of trace
  • We do this by moving the trace docs into the tutorial, raising visibility
  • This provides us an opportunity to do what I've been meaning to for a while: reduce our top-level modules further by merging trace into combinator
  • Highlight the tutorial in _topics
• make cut_err more accessible
  • We do this by improving the cross-linking
  • Highlight the tutorial in _topics

[coveralls]

Pull Request Test Coverage Report for Build 7660463966

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 43.28%

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Totals
Change from base Build 7594344864:	-0.1%
Covered Lines:	1256
Relevant Lines:	2902

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💛 - Coveralls