docs/conf.py: add autodoc_mock_imports for klein and twisted
The eSIM SM-DP+ server modules (`pySim.esim.es2p`, `pySim.esim.es9p`, `pySim.esim.http_json_api`) unconditionally import optional server-side dependencies at module level:
pySim.esim.es2p -- from klein import Klein pySim.esim.http_json_api -- from twisted.web.server import Request
Both imports fail during a docs build if the packages are absent or broken, causing three "autodoc: failed to import" warnings and three missing chapters in the generated manual.
Even when klein and twisted are installed, twisted 23.10.0 (the version pulled in transitively by smpp.twisted3's `Twisted~=23.10.0` constraint) is incompatible with Python 3.13+ because twisted.web.http unconditionally executes `import cgi`, a module that was removed from the standard library in Python 3.13.
Fix: add `autodoc_mock_imports = ['klein', 'twisted']` to conf.py. Sphinx inserts mock entries into sys.modules before each autodoc import attempt, so the modules can be imported and documented without requiring the real packages to be importable at build time.
