Update tutorial on adding new extensions

[duckontheweb]

Related Issue(s):

• Closes Update tutorial for adding new/custom extensions #384

Description:

Updates the "Adding New and Custom Extensions" tutorial to be up-to-date with the new extensions API. The tutorial covers implementation of a PropertiesExtension sub-class that operates on Items, but does not cover summaries (a separate tutorial on creating summaries in general may be in order anyway).

Also updates the order of the TOC in the tutorials* pages to match the order of the tutorials in the index page.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #724 (3495885) into main (351f10f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #724   +/-   ##
=======================================
  Coverage   94.33%   94.33%           
=======================================
  Files          77       77           
  Lines       11285    11285           
  Branches     1347     1347           
=======================================
  Hits        10646    10646           
  Misses        459      459           
  Partials      180      180           

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[schwehr]

Only gave it a light read.

[duckontheweb]

@lossyrob @matthewhanson I'll plan on merging this by the end of today if there isn't any additional feedback.