CI: added github pages build job

mkdocs.yml

@@ -0,0 +1,43 @@
+site_name: SDL
+site_description: Simple Direct Media Layer
+docs_dir: docs
+site_dir: build/docs
+repo_url: https://github.com/libsdl-org/SDL
+repo_name: GitHub
+
+theme:
+  name: readthedocs
+  highlightjs: true
+  hljs_languages: [c]
+
+markdown_extensions:
+  - toc:
+      toc_depth: 2
+
+nav:
+  - README.md
+  - README-android.md
+  - README-cmake.md
+  - README-directfb.md
+  - README-dynapi.md
+  - README-emscripten.md
+  - README-gesture.md
+  - README-git.md
+  - README-hg.md
+  - README-ios.md
+  - README-kmsbsd.md
+  - README-linux.md
+  - README-macosx.md
+  - README.md
+  - README-nacl.md
+  - README-os2.md
+  - README-pandora.md
+  - README-platforms.md
+  - README-porting.md
+  - README-psp.md
+  - README-raspberrypi.md
+  - README-touch.md
+  - README-visualc.md
+  - README-wince.md
+  - README-windows.md
+  - README-winrt.md