chore: convert changelog to markdown

[henryiii]

Simpler changelog blocks in pull requests, and simpler in creating the GitHub releases.

Removed the mention of the upgrade guide as I think not one single person has ever added anything in that block.

• chore: move changelog to markdown
• chore: switch to simpler block def

---

📚 Documentation preview 📚: https://pybind11--5672.org.readthedocs.build/

[henryiii]

3.14t macOS.

______________ test_run_in_process_direct[_intentional_deadlock] _______________

test_fn = <function _intentional_deadlock at 0x2898f7a4740>

    @pytest.mark.skipif(sys.platform.startswith("emscripten"), reason="Requires threads")
    @pytest.mark.parametrize("test_fn", ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK)
    @pytest.mark.skipif(
        "env.GRAALPY",
        reason="GraalPy transiently complains about unfinished threads at process exit",
    )
    def test_run_in_process_direct(test_fn):
        """Makes sure there is no GIL deadlock when using processes.
    
        This test is for completion, but it was never an issue.
        """
>       assert _run_in_process(test_fn) == 0

test_fn    = <function _intentional_deadlock at 0x2898f7a4740>

test_gil_scoped.py:269: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

target = <function _intentional_deadlock at 0x2898f7a4740>, args = ()
kwargs = {}, test_fn = <function _intentional_deadlock at 0x2898f7a4740>
timeout = 0.1
process = <Process name='Process-72' pid=5355 parent=5115 stopped exitcode=0 daemon>
t_start = 1747718249.89369, t_delta = 0.10827970504760742, @py_assert1 = 0
@py_assert4 = None, @py_assert3 = False
@py_format6 = "0\n{0 = <Process name='Process-72' pid=5355 parent=5115 stopped exitcode=0 daemon>.exitcode\n} is None"
@py_format8 = "assert 0\n{0 = <Process name='Process-72' pid=5355 parent=5115 stopped exitcode=0 daemon>.exitcode\n} is None"

    def _run_in_process(target, *args, **kwargs):
        test_fn = target if len(args) == 0 else args[0]
        # Do not need to wait much, 10s should be more than enough.
        timeout = 0.1 if test_fn is _intentional_deadlock else 10
        process = multiprocessing.Process(target=target, args=args, kwargs=kwargs)
        process.daemon = True
        try:
            t_start = time.time()
            process.start()
            if timeout >= 100:  # For debugging.
                print(
                    "\nprocess.pid STARTED", process.pid, (sys.argv, target, args, kwargs)
                )
                print(f"COPY-PASTE-THIS: gdb {sys.argv[0]} -p {process.pid}", flush=True)
            process.join(timeout=timeout)
            if timeout >= 100:
                print("\nprocess.pid JOINED", process.pid, flush=True)
            t_delta = time.time() - t_start
            if process.exitcode == 66 and m.defined_THREAD_SANITIZER:  # Issue #2754
                # WOULD-BE-NICE-TO-HAVE: Check that the message below is actually in the output.
                # Maybe this could work:
                # https://gist.github.com/alexeygrigorev/01ce847f2e721b513b42ea4a6c96905e
                pytest.skip(
                    "ThreadSanitizer: starting new threads after multi-threaded fork is not supported."
                )
            elif test_fn is _intentional_deadlock:
>               assert process.exitcode is None
E               AssertionError: assert 0 is None
E                +  where 0 = <Process name='Process-72' pid=5355 parent=5115 stopped exitcode=0 daemon>.exitcode

args       = ()
kwargs     = {}
process    = <Process name='Process-72' pid=5355 parent=5115 stopped exitcode=0 daemon>
t_delta    = 0.10827970504760742
t_start    = 1747718249.89369
target     = <function _intentional_deadlock at 0x2898f7a4740>
test_fn    = <function _intentional_deadlock at 0x2898f7a4740>
timeout    = 0.1

test_gil_scoped.py:187: AssertionError
=============================== warnings summary ===============================
<frozen importlib._bootstrap>:491
  <frozen importlib._bootstrap>:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'exo_planet_c_api', which has not declared that it can run safely without the GIL. To override this behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[Copilot]

Pull Request Overview

This PR converts changelog sections from reStructuredText to Markdown and updates references accordingly.

• Renames the changelog entry regex and adjusts its pattern for Markdown.
• Updates output formatting to use Markdown syntax and switches file references from .rst to .md.
• Updates GitHub configuration files to recognize the new changelog file.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/make_changelog.py	Renames regex and prints entries in Markdown, handles legacy RST entries separately.
docs/release.rst	Updates release documentation to reference the Markdown changelog.
.github/pull_request_template.md	Adjusts the pull request template to accommodate the new changelog format.
.github/labeler_merged.yml	Updates file globs to target the Markdown changelog.
.github/labeler.yml	Adjusts labeler configuration to ignore the Markdown changelog.

Comments suppressed due to low confidence (1)

tools/make_changelog.py:34

• [nitpick] Consider renaming the variable 'old' to a more descriptive name such as 'old_rst_entries' to clarify its purpose.

old = []

[henryiii]

I also would like to grep and fix PYBIND11_GLOBAL_, it's, and setup.py, but will do it in a followup. We did lose the contents of .. <...>, but I don't think it really matters, it's just a grouping.