Update the types tutorial

[konnov]

Closes #1939. This PR update the tutorial on the type checker, using the new records and giving pointers to variants. I have made it shorter and, hopefully, simpler.

• Entries added to ./unreleased/ for any new functionality

[codecov-commenter]

Codecov Report

Merging #1942 (306a0cc) into unstable (caec3b4) will not change coverage.
The diff coverage is n/a.

❗ Current head 306a0cc differs from pull request most recent head b582b34. Consider uploading reports for the commit b582b34 to get more accurate results

@@            Coverage Diff            @@
##           unstable    #1942   +/-   ##
=========================================
  Coverage     77.09%   77.09%           
=========================================
  Files           415      415           
  Lines         12676    12676           
  Branches        568      568           
=========================================
  Hits           9772     9772           
  Misses         2904     2904           

Impacted Files	Coverage Δ
...a/at/forsyte/apalache/tla/lir/TlaLevelFinder.scala	97.36% <0.00%> (-2.64%)	⬇️
...he/io/annotations/parser/CommentPreprocessor.scala	97.80% <0.00%> (+1.09%)	⬆️

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[thpani]

Claiming the review token – I agreed with @konnov to push minor improvements directly to this branch.

[thpani]

LGTM 👍

I've pushed some suggestions for minor improvements in 25f9dce, 6f8e0e5, 50e64b8. @konnov please check if you think they're helpful.