Fix missing directory of document for new tag v0.9.0 (#776)

The v0.9.0 conf.py (introduced in #775) dynamically loads the version
from python/mscclpp/_version.py.

This file is generated at build time by setuptools_scm and is listed in
.gitignore — it is never committed to the repo. Earlier tags (v0.8.0 and
below) used a hardcoded release (e.g., "v0.8.0") in conf.py, so they had
no dependency on generated files.
sphinx-multiversion checks out each tag using git archive, which only
extracts committed files.
Since _version.py is not committed, the v0.9.0 checkout is missing it,
and conf.py crashes on import. All future tags will have this same
problem.

**Three changes:**
1. docs/build_multiversion.py (new): A wrapper around
sphinx-multiversion that monkey-patches copy_tree to generate
_version.py in each tag checkout after extraction. The version string is
parsed from the tag name (e.g., v0.9.0 → __version__ = "0.9.0").
2. Makefile: The multiversion target now calls build_multiversion.py
instead of sphinx-multiversion directly.
3. conf.py: Added a fallback so that if _version.py doesn't exist, it
reads the version from the VERSION file instead. This makes conf.py
resilient for any future scenario where _version.py is missing.

**Testing**
Verified locally:
• make multiversion now successfully builds all 11 versions (v0.4.0
through v0.9.0)
• v0.9.0 docs are correctly generated under _build/html/v0.9.0/
Version selector shows v0.9.0 as latest

docs/Makefile

@@ -5,7 +5,7 @@
 # from the environment for the first two.
 SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
-SPHINXMULTIVERSION ?= sphinx-multiversion
+SPHINXMULTIVERSION ?= python3 build_multiversion.py
 SOURCEDIR     = .
 BUILDDIR      = _build
 

docs/build_multiversion.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+"""Wrapper around sphinx-multiversion that patches copy_tree to generate
+_version.py in each tag checkout. This is needed because setuptools_scm
+generates _version.py at build time, but sphinx-multiversion uses
+`git archive` which only contains committed files.
+
+Usage (called by Makefile):
+    python3 build_multiversion.py <sourcedir> <outputdir> [sphinx-opts...]
+"""
+
+import os
+import re
+import subprocess
+import sys
+
+import sphinx_multiversion.git as smv_git
+from sphinx_multiversion import main as smv_main
+
+# Save the original copy_tree
+_original_copy_tree = smv_git.copy_tree
+
+
+def _patched_copy_tree(gitroot, src, dst, reference, sourcepath="."):
+    """Call original copy_tree, then generate _version.py from the VERSION file."""
+    _original_copy_tree(gitroot, src, dst, reference, sourcepath)
+
+    # Extract version from the tag name (e.g., "v0.9.0" -> "0.9.0")
+    refname = getattr(reference, "refname", "") or ""
+    match = re.search(r"v(\d+\.\d+\.\d+)", refname)
+    if not match:
+        return
+
+    version = match.group(1)
+    version_py_dir = os.path.join(dst, "python", "mscclpp")
+    if os.path.isdir(version_py_dir):
+        version_py = os.path.join(version_py_dir, "_version.py")
+        if not os.path.exists(version_py):
+            with open(version_py, "w") as f:
+                f.write(f'__version__ = "{version}"\n')
+
+
+# Monkey-patch
+smv_git.copy_tree = _patched_copy_tree
+
+if __name__ == "__main__":
+    sys.exit(smv_main(sys.argv[1:]))