Document using Requires.jl for extending in 3rd-party packages

[MichaelHatherly]

Since packages that would want to extend QNW would need QuartoNotebookWorker in their weakdeps and that requires that the weakdep be registered for registry CI checks to pass. Extending with Requires.jl does not have that issue since it is dynamic rather than based on what the manifest resolves to.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.43%. Comparing base (85f6510) to head (ef91163).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   53.43%   53.43%           
=======================================
  Files          27       27           
  Lines        1209     1209           
=======================================
  Hits          646      646           
  Misses        563      563           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.