Add Python API documentation generation with pdoc3

This commit adds automated Python API documentation generation for all
workspace packages using pdoc3:

- Add contrib/api/generate-python-docs.py script to generate docs
- Add Makefile targets: python-docs and python-docs-clean
- Add GitHub Actions workflow for nightly documentation generation
- Documents 5 packages: pyln.client, pyln.proto, pyln.grpc, pyln.testing, pyln.spec.bolt7
- Creates beautiful index page with cards linking to each package
- Stores generated docs as artifacts with 90-day retention
- Add pdoc3 and markdown to dev dependencies

Bug fix:
- Fix pyln-client version.py: __all__ must contain strings, not class objects
  This was causing "TypeError: attribute name must be string, not 'type'" in pdoc3

Documentation is generated to docs/python/ which is excluded from version control.
Run 'make python-docs' to generate locally, or download from nightly workflow artifacts.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

pyproject.toml

@@ -24,7 +24,7 @@ package-mode = false
 [dependency-groups]
 dev = [
   # Test dependencies and inherited dependencies belong here
-  "crc32c>=2.2.post0",              # Belongs to lnprototest
+  "crc32c>=2.2.post0", # Belongs to lnprototest
   "pytest>=8.0.0",
   "pytest-xdist>=3.6.0",
   "pytest-test-groups>=1.2.0",
@@ -35,6 +35,7 @@ dev = [
   "flask-socketio>=5",
   "tqdm",
   "pytest-benchmark",
+  "pdoc3>=0.11.6",
 ]
 
 [project.optional-dependencies]