Fix broken @extref links breaking the docs build

Two external cross-references no longer resolve against the upstream
inventories, and makedocs treats :external_cross_references as fatal, so
the Documentation job fails before rendering:

- JuliaFormatter registers `format` as a jl:function with no signature
  suffix, so `JuliaFormatter.format-Tuple{Any}` is gone.
- PowerSystems renamed the time-series tutorial page, dropping the
  `tutorial_time_series` label.

Also convert a hard-coded URL to the same (renamed, now 404ing)
PowerSystems tutorial into an @extref, per the guidance in
general_formatting.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: daniel-thom

docs/src/docs_best_practices/how-to/troubleshoot.md

@@ -82,7 +82,7 @@ Pkg.add(["PowerSystems"]);
      2. Run the formatter once to format the rest of the file
      3. Add the text back in
      4. Add the `ignore` keyword argument with the file name to
-        [`JuliaFormatter.format`](@extref `JuliaFormatter.format-Tuple{Any}`) in `scripts/formatter/formatter_code.jl`
+        [`JuliaFormatter.format`](@extref JuliaFormatter.format) in `scripts/formatter/formatter_code.jl`
         to skip the file in the future:
 
 ```julia

docs/src/docs_best_practices/how-to/write_a_tutorial.md

@@ -10,7 +10,7 @@ Sienna.
     the [difference between a tutorial and how-to guide](https://diataxis.fr/tutorials-how-to/)
     to refresh your memory and refer back throughout the process.
   - Look at an example: `PowerSystems.jl`'s
-    [Working with Time Series](https://sienna-platform.github.io/PowerSystems.jl/stable/tutorials/working_with_time_series/)
+    [Working with Time Series](@extref PowerSystems :doc:`tutorials/generated_working_with_time_series`)
 
 !!! warning
 
@@ -56,7 +56,7 @@ code blocks.
 !!! tip "Do"
 
     Display all code, starting from `using SomeSiennaPackage`. Example: See
-    [Working with Time Series](@extref tutorial_time_series).
+    [Working with Time Series](@extref PowerSystems :doc:`tutorials/generated_working_with_time_series`).
 
 !!! warning "Don't"