ci: add documentation build check

Add a docs-check workflow, calling the reusable check-library-docs
workflow from stm32duino/actions, to build the documentation with
Doxygen and MkDocs in strict mode on pull requests and pushes to main
that touch the documentation sources.

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>
Assisted-by: GitHub Copilot:claude-sonnet-5.5

Author: fpistm

.github/workflows/docs-check.yml

@@ -0,0 +1,26 @@
+name: Documentation build
+
+on:
+  pull_request:
+    paths:
+      - Doxyfile
+      - docs/**
+      - src/**
+      - mkdocs.yml
+      - .github/workflows/docs-check.yml
+  push:
+    branches:
+      - main
+    paths:
+      - Doxyfile
+      - docs/**
+      - src/**
+      - mkdocs.yml
+      - .github/workflows/docs-check.yml
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    uses: stm32duino/actions/.github/workflows/check-library-docs.yml@main