[Docs] CLI command to serve docs locally (#6956)

* CLI command to serve docs locally

* Document it

* Default port

* Use `with` and subclass `SimpleHTTPRequestHandler` to set working dir

* Apply suggestions from code review

Co-Authored-By: skullydazed <skullydazed@users.noreply.github.com>

* Update docs/cli.md

lib/python/qmk/cli/__init__.py

@@ -5,6 +5,7 @@ We list each subcommand here explicitly because all the reliable ways of searchi
 from . import cformat
 from . import compile
 from . import config
+from . import docs
 from . import doctor
 from . import hello
 from . import json

lib/python/qmk/cli/docs.py

@@ -0,0 +1,22 @@
+"""Serve QMK documentation locally
+"""
+import http.server
+
+from milc import cli
+
+
+class DocsHandler(http.server.SimpleHTTPRequestHandler):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, directory='docs', **kwargs)
+
+
+@cli.argument('-p', '--port', default=8936, type=int, help='Port number to use.')
+@cli.subcommand('Run a local webserver for QMK documentation.')
+def docs(cli):
+    """Spin up a local HTTPServer instance for the QMK docs.
+    """
+    with http.server.HTTPServer(('', cli.config.docs.port), DocsHandler) as httpd:
+        cli.log.info("Serving QMK docs at http://localhost:%d/", cli.config.docs.port)
+        cli.log.info("Press Control+C to exit.")
+
+        httpd.serve_forever()