fix: chapters in PDF again (#2606)

* fix: chapters in PDF again

* fix: sections in README

README.rst

 .. figure:: https://github.com/pybind/pybind11/raw/master/docs/pybind11-logo.png
    :alt: pybind11 logo
 
-pybind11 — Seamless operability between C++11 and Python
-========================================================
+**pybind11 — Seamless operability between C++11 and Python**
 
 |Latest Documentation Status| |Stable Documentation Status| |Gitter chat| |CI| |Build status|
 
@@ -53,9 +52,11 @@ A PDF version of the manual is available
 And the source code is always available at
 `github.com/pybind/pybind11 <https://github.com/pybind/pybind11>`_.
 
+
 Core features
 -------------
 
+
 pybind11 can map the following core C++ features to Python:
 
 - Functions accepting and returning custom data structures per value,

docs/conf.py

@@ -17,6 +17,10 @@ import sys
 import os
 import shlex
 import subprocess
+from pathlib import Path
+import re
+
+DIR = Path(__file__).parent.resolve()
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -345,6 +349,29 @@ def generate_doxygen_xml(app):
         sys.stderr.write("doxygen execution failed: {}\n".format(e))
 
 
+def prepare(app):
+    with open(DIR.parent / "README.rst") as f:
+        contents = f.read()
+
+    # Filter out section titles for index.rst for LaTeX
+    if app.builder.name == "latex":
+        contents = re.sub(r"^(.*)\n[-~]{3,}$", r"**\1**", contents, flags=re.MULTILINE)
+
+    with open(DIR / "readme.rst", "w") as f:
+        f.write(contents)
+
+
+def clean_up(app, exception):
+    (DIR / "readme.rst").unlink()
+
+
 def setup(app):
-    """Add hook for building doxygen xml when needed"""
+
+    # Add hook for building doxygen xml when needed
     app.connect("builder-inited", generate_doxygen_xml)
+
+    # Copy the readme in
+    app.connect("builder-inited", prepare)
+
+    # Clean up the generated readme
+    app.connect("build-finished", clean_up)

docs/index.rst

@@ -3,7 +3,7 @@
    Intro
    =====
 
-.. include:: ../README.rst
+.. include:: readme.rst
 
 .. only:: not latex