documentation: add two hints to MLIR introduction notebook

[tobiasgrosser]

Some of these questions are actually not super easy. Let's not discourage people trying out our notebook at the very first exercise.

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.15%. Comparing base (e4048ab) to head (fd0bb3f).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
docs/marimo/mlir_introduction.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5599   +/-   ##
=======================================
  Coverage   86.15%   86.15%           
=======================================
  Files         393      393           
  Lines       56302    56304    +2     
  Branches     6492     6492           
=======================================
+ Hits        48507    48509    +2     
- Misses       6262     6263    +1     
+ Partials     1533     1532    -1     

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