ci: add documentation build job

Runs api-docs (Doxygen → AsciiDoc), docs-latex (AsciiDoc → LaTeX via
asciidoctor), and math-companion (LaTeX → PDF) in a dedicated job
parallel to builds and Isabelle verification. Installs doxygen, Ruby +
asciidoctor-latex gem, and texlive. Uploads docs as a Gitea artifact
on every run regardless of pass/fail.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -212,3 +212,39 @@ jobs:
         with:
           name: isabelle-proof-log-${{ github.sha }}
           path: /tmp/isabelle-build.log
+
+  build-docs:
+    name: Documentation
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install doc toolchain
+        run: |
+          apt-get update -qq
+          apt-get install -y --no-install-recommends \
+            doxygen python3 \
+            ruby ruby-dev build-essential \
+            texlive-latex-base texlive-latex-extra \
+            texlive-science texlive-fonts-recommended
+          gem install asciidoctor asciidoctor-latex --no-document
+
+      - name: API docs (Doxygen → AsciiDoc)
+        run: make api-docs
+
+      - name: LaTeX docs (AsciiDoc → LaTeX)
+        run: make docs-latex
+
+      - name: Math companion PDF (LaTeX → PDF)
+        run: make math-companion
+
+      - name: Upload documentation
+        if: always()
+        uses: actions/upload-artifact@v3
+        with:
+          name: docs-${{ github.sha }}
+          path: |
+            docs/src/appendix/
+            docs/latex/
+            docs/SSRN_companion/Math_Companion_SSRN.pdf