Update user guide

[liamhuber]

Adds sections for parsing constants and item/attr access, and highlights the stdlib a little bit. Promotion of the most important parts of the API pre-empty #295 to stop mentioning parse_workflow -- it was anyhow the only method listed there that was not used in the rest of the notebook!

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[github-actions]

👈 Launch a binder notebook on branch pyiron/flowrep/update-user-guide

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.97%. Comparing base (0884db7) to head (9cce0ea).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           remove-dataclass-unpack     #297   +/-   ##
========================================================
  Coverage                    99.97%   99.97%           
========================================================
  Files                           43       43           
  Lines                         3474     3474           
========================================================
  Hits                          3473     3473           
  Misses                           1        1           

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