noxfile: add make session

I know calling make via nox is a bit weird, but I think allowing to run
the Makefile in an isolated environment without having to manually
manage venvs and making it easy to choose between relaxed and stable
dependencies is quite useful.

We should eventually move away from the Makefile, but I think some level
of integration is better than nothing.
Translating the Makefile syntax and constructs into nox's Python syntax
and untangling all of the different Python scripts that pre process the
documentation is a longer term project.

noxfile.py

@@ -4,6 +4,7 @@
 from argparse import ArgumentParser, BooleanOptionalAction
 from glob import iglob
 from pathlib import Path
+from typing import cast
 
 import nox
 
@@ -121,11 +122,9 @@ def clone_core(session: nox.Session):
 ]
 
 
-@nox.session
-@nox.parametrize(["test"], checker_tests, checker_tests)
-def checkers(session: nox.Session, test: str):
+def _relaxed_parser(session: nox.Session) -> ArgumentParser:
     """
-    Run docs build checkers
+    Generate an argument parser with a --relaxed option.
     """
     parser = ArgumentParser(prog=f"nox -e {session.name} --")
     parser.add_argument(
@@ -134,8 +133,42 @@ def checkers(session: nox.Session, test: str):
         action=BooleanOptionalAction,
         help="Whether to use requirements-relaxed file. (Default: %(default)s)",
     )
-    args = parser.parse_args(session.posargs)
+    return parser
+
+
+def _env_python(session: nox.Session) -> str:
+    """
+    Get the full path to an environment's python executable
+    """
+    out = cast(
+        str,
+        session.run("python", "-c", "import sys; print(sys.executable)", silent=True),
+    )
+    return out.strip()
+
+
+@nox.session
+@nox.parametrize(["test"], checker_tests, checker_tests)
+def checkers(session: nox.Session, test: str):
+    """
+    Run docs build checkers
+    """
+    args = _relaxed_parser(session).parse_args(session.posargs)
 
     install(session, req="requirements-relaxed" if args.relaxed else "requirements")
     session.run("make", "-C", "docs/docsite", "clean", external=True)
     session.run("python", "tests/checkers.py", test)
+
+
+@nox.session
+def make(session: nox.Session):
+    """
+    Run the docs build Makefile in an isolated environment
+    """
+    parser = _relaxed_parser(session)
+    parser.add_argument("make_args", nargs="*", help="Arguments to pass on to make")
+    args = parser.parse_args(session.posargs)
+
+    install(session, req="requirements-relaxed" if args.relaxed else "requirements")
+    make_args: list[str] = [f"PYTHON={_env_python(session)}"] + args.make_args
+    session.run("make", "-C", "docs/docsite", *make_args, external=True)