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.

Change-Id: I71650466f02a6a6d150650deed167c05d2cb6e64

docs/conf.py

@@ -65,6 +65,10 @@ html_static_path = ['_static']
 
 autoclass_content = 'both'
 
+# Mock optional server-side deps of es2p and http_json_api/es9p,
+# so that autodoc can import and document those modules.
+autodoc_mock_imports = ['klein', 'twisted']
+
 # Workaround for duplicate label warnings:
 # https://github.com/sphinx-doc/sphinx-argparse/issues/14
 #