Enable using PULPDOCS_DIR to specify pulp-docs checkout location

Author: pedro-psb

src/pulp_docs/cli.py

@@ -50,3 +50,6 @@ def draft_callback(ctx: click.Context, param: click.Parameter, value: bool) -> b
     draft_option(sub_command)
     blog_option(sub_command)
     docstrings_option(sub_command)
+    serve_options = sub_command.params
+    config_file_opt = next(filter(lambda opt: opt.name == "config_file", serve_options))
+    config_file_opt.envvar = "PULPDOCS_DIR"