docs: fix py::trampoline_self_life_support visibility in docs

[gentlegiantJGC]

Description

The documentation examples for py::trampoline_self_life_support inherit it privately which I belive is incorrect.

Suggested changelog entry:

• Fix py::trampoline_self_life_support visibility in docs

---

📚 Documentation preview 📚: https://pybind11--5766.org.readthedocs.build/

[rwgk]

Thanks for catching these!

[rwgk]

@b-pass this flake looks a little worrying:

https://github.com/pybind/pybind11/actions/runs/16496837953/job/46644528777?pr=5766

Copy-pasting from the CI log, for future reference:

> Run cmake --build build --target cpptest

Change Dir: '/home/runner/work/pybind11/pybind11/build'

Run Build Command(s): /usr/local/bin/ninja -v cpptest
[0/2] /usr/local/bin/cmake -P /home/runner/work/pybind11/pybind11/build/CMakeFiles/VerifyGlobs.cmake
[1/2] cd /home/runner/work/pybind11/pybind11/build/tests/test_embed && /home/runner/work/pybind11/pybind11/build/tests/test_embed/test_embed
FAILED: [code=139] tests/test_embed/CMakeFiles/cpptest /home/runner/work/pybind11/pybind11/build/tests/test_embed/CMakeFiles/cpptest 
cd /home/runner/work/pybind11/pybind11/build/tests/test_embed && /home/runner/work/pybind11/pybind11/build/tests/test_embed/test_embed
Exception ignored in: <module 'threading' from '/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/threading.py'>
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/threading.py", line 1602, in _shutdown
    assert tlock.locked()
           ^^^^^^^^^^^^^^
AssertionError: 
Debug memory block at address p=0x7f86bc10f200: API 'E'
    4642792059332132864 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfd):
        at p-7: 0x00 *** OUCH
        at p-6: 0x00 *** OUCH
        at p-5: 0x00 *** OUCH
        at p-4: 0x00 *** OUCH
        at p-3: 0x00 *** OUCH
        at p-2: 0x00 *** OUCH
        at p-1: 0x00 *** OUCH
    Because memory is corrupted at the start, the count of bytes requested
       may be bogus, and checking the trailing pad bytes may segfault.
    The 8 pad bytes at tail=0x406f024e428ff200 are 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test_embed is a Catch v2.13.10 host application.
Run with -? for options

-------------------------------------------------------------------------------
Per-Subinterpreter GIL
-------------------------------------------------------------------------------
/home/runner/work/pybind11/pybind11/tests/test_embed/test_subinterpreter.cpp:303
...............................................................................

/home/runner/work/pybind11/pybind11/tests/test_embed/test_subinterpreter.cpp:303: FAILED:
  {Unknown expression after the reported line}
due to a fatal error condition:
  SIGSEGV - Segmentation violation signal

===============================================================================
test cases:   20 |   19 passed | 1 failed
assertions: 1588 | 1587 passed | 1 failed

Fatal Python error: Segmentation fault

Current thread 0x00007f86b1ffd6c0 (most recent call first):
  File "<frozen importlib._bootstrap_external>", line 1652 in _fill_cache
  File "<frozen importlib._bootstrap_external>", line 1609 in find_spec
  File "<frozen importlib._bootstrap_external>", line 1506 in _get_spec
  File "<frozen importlib._bootstrap_external>", line 1532 in find_spec
  File "<frozen importlib._bootstrap>", line 1262 in _find_spec
  File "<frozen importlib._bootstrap>", line 1322 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1360 in _find_and_load
  File "<frozen importlib._bootstrap>", line 488 in _call_with_frames_removed
  File "<frozen importlib._bootstrap>", line 1415 in _handle_fromlist
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/encodings/__init__.py", line 33 in <module>
  File "<frozen importlib._bootstrap>", line 488 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 999 in exec_module
  File "<frozen importlib._bootstrap>", line 935 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1331 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1360 in _find_and_load
Segmentation fault (core dumped)
ninja: build stopped: subcommand failed.

Error: Process completed with exit code 139.