Fix missing directory of document for new tag v0.9.0

[seagater]

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