html-proofer baseline debt: accel.md screenshot + signals notebook alt

[jonfroehlich]

html-proofer baseline debt (ignored, tracked)

The new html-proofer link-check gate (.github/workflows/content-lint.yml) is green only because these pre-existing gaps are on its --ignore-files list. Tracking them here so the ignores are intentional, not silent:

Draft page, missing screenshot

• arduino/accel.md (draft, nav_exclude) references assets/images/ArduinoIDE_LibraryManager_AdafruitLIS3DH.png, which doesn't exist. Add the screenshot or remove the reference, then drop /arduino/accel.html from the ignore list.

Generated notebook HTML missing alt text

• The stale nbconvert notebooks under signals/*/index.html (ComparingSignals, FeatureSelectionAndHyperparameterTuning, FrequencyAnalysis, QuantizationAndSampling, StepTracker) contain matplotlib output_*.png outputs with no alt. These are auto-generated, pandemic-era static HTML and are ignored wholesale. Revisit if/when the signals notebooks are reworked (see V2.0 framework discussion). Drop /signals/.+/index.html from the ignore list when addressed.

Each ignore entry in the workflow points back to this issue.

Part of V2.0.