Fix broken @extref links breaking the docs build

[daniel-thom]

Problem

The Documentation CI job has been failing on main since run 27879924041 (2026-06-20), and on every open PR branch as a result. Two @extref (DocumenterInterLinks) references no longer resolve against the upstream inventories, and makedocs treats :external_cross_references as fatal:

ERROR: LoadError: `makedocs` encountered an error [:external_cross_references] -- terminating build before rendering.

Because the failure happens in the external-link phase, the build terminates before rendering — nothing else in the CI log is a real signal.

Cause and fix

File	Was	Why it broke	Now
docs_best_practices/how-to/troubleshoot.md	@extref `JuliaFormatter.format-Tuple{Any}`	JuliaFormatter's inventory registers format as a jl:function with no signature suffix	@extref JuliaFormatter.format
docs_best_practices/how-to/write_a_tutorial.md	@extref tutorial_time_series	PowerSystems renamed the page (Literate-generated now), dropping the label	@extref PowerSystems :doc:`tutorials/generated_working_with_time_series`

Also converts a hard-coded URL to that same PowerSystems tutorial (write_a_tutorial.md:13) into an @extref. It currently 404s for the same rename reason — Documenter doesn't linkcheck it, so it wasn't a build failure, just a dead link. general_formatting.md in this same guide tells authors to prefer @extref over hard-coded URLs.

Verification

julia --project=docs docs/make.jl exits 0 with zero cannot resolve external link errors. Confirmed the rendered HTML points at live URLs:

• https://juliaeditorsupport.github.io/JuliaFormatter.jl/stable/api/#JuliaFormatter.format
• https://sienna-platform.github.io/PowerSystems.jl/stable/tutorials/generated_working_with_time_series/

Remaining build warnings (unbalanced $ in InfrastructureSystems.md, the page-size warning) are pre-existing and non-fatal.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.86%. Comparing base (9af896f) to head (8429356).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #598      +/-   ##
==========================================
- Coverage   78.88%   78.86%   -0.02%     
==========================================
  Files          75       75              
  Lines        6660     6649      -11     
==========================================
- Hits         5254     5244      -10     
+ Misses       1406     1405       -1     

Flag	Coverage Δ
unittests	78.86% <ø> (-0.02%)	⬇️

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see 6 files with indirect coverage changes

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