Add extension to remove YAML front matter from includes

Introduced a custom Sphinx extension 'include_remove_yaml' to strip YAML front matter from included files. Updated _config.yml to register the extension, and added YAML front matter to intro.md for demonstration.

book/_config.yml

@@ -11,6 +11,8 @@ html:
   baseurl  : "https://teachbooks.io/manual/"
 
 sphinx:
+  local_extensions:
+    include_remove_yaml: _ext/
   extra_extensions:
   - sphinx_inline_tabs
   - sphinx.ext.imgconverter

book/_ext/include_remove_yaml/__init__.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+from sphinx.util import logging
+logger = logging.getLogger(__name__)
+
+def _strip_yaml(lines: list[str]) -> list[str]:
+
+    if not lines:
+        return lines
+    for ix,lines_set in enumerate(lines):
+        new_lines = lines_set.splitlines()
+        if new_lines[0].strip() != "---":
+            continue
+        # yaml front matter detected
+        # find closing delimiter
+        index_end = 1
+        for i in range(1, len(new_lines)):
+            tok = new_lines[i].strip()
+            if tok == "---":
+                index_end = i
+                break
+        new_lines = new_lines[index_end+1 :]
+        lines[ix] = "\n".join(new_lines)
+    return lines  # no closing delimiter -> leave unchanged
+
+def on_include_read(app, docname, path, content):
+    # content is a list[str]; modify in-place
+    new = _strip_yaml(content)
+    if new is not content:
+        content[:] = new
+
+def setup(app):
+    app.connect("include-read", on_include_read)
+    return {}

book/intro.md

@@ -1,3 +1,8 @@
+---
+file_format: mystnb
+kernelspec:
+  name: python3
+---
 # TeachBooks Manual
 
 This manual is primarily designed for and by teachers for use in education, but should be a useful resource for anyone interested in creating and collaborating on a TeachBook: a <a href="https://jupyterbook.org/v1/"><img  style="display:inline-block; height:1.5em; width:auto; transform:translate(0, -0.15em)" src="images/logo-wide.svg" alt="Jupyter book v1"> v1</a> using the <a href="https://www.sphinx-doc.org/"><img  style="display:inline-block; height:1.5em; width:auto; transform:translate(0, -0.15em)" src="images/sphinx-logo.svg" alt="Sphinx logo"> Sphinx backend</a> with additional editing tools and book features. Our aim is to provide a simple way to start book-making for new users (it only takes 10 clicks!), ease the editing process for anyone, and provide additional features developed for education-purposes. We hope you find this resource useful and refer back to it often.