docs(PRT): add PRT introductory notebooks

[wpbonelli]

• [x] PRT introduction tutorial notebook
  • create simple structured model based on flopy readme
• [x] migrating from MODPATH 7 to PRT example notebook
  • create then compare results from identical simulations based on flopy readme, first structured then vertex grid
• [ ] update some mp7 notebooks with identical PRT models?

Notes

• isolated from #1753
• temporary CI workarounds to update flopy classes, can remove after 1752 merges and before merging
• notebooks also depend on PRT package classes coming separately, workarounds can be removed after #1921

[codecov[bot]]

Codecov Report

Merging #1940 (7896286) into develop (4f6cd47) will increase coverage by 0.0%. Report is 1 commits behind head on develop. The diff coverage is n/a.

:exclamation: Current head 7896286 differs from pull request most recent head 4c582fd. Consider uploading reports for the commit 4c582fd to get more accurate results

@@           Coverage Diff           @@
##           develop   #1940   +/-   ##
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  Coverage     72.6%   72.6%           
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  Files          257     257           
  Lines        57689   57699   +10     
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+ Hits         41905   41915   +10     
  Misses       15784   15784           

see 1 file with indirect coverage changes

[wpbonelli]

probably not necessary given the PRT examples in the modflow6-examples repo