[docs] Exclude unrelated files

`apps/` and `post-processing/` are symlinks to the directories in our
repository, with the result the website includes *all* the files in them.

mkdocs.yml

@@ -53,3 +53,10 @@ markdown_extensions:
   - pymdownx.inlinehilite
   - pymdownx.snippets
   - pymdownx.superfences
+exclude_docs: |
+  # Keep only HTML files under `apps`
+  apps/**
+  !apps/**/*.html
+  # Keep only HTML files under `post-processing`
+  post-processing/**
+  !post-processing/**/*.html