tools: Add tools/build-docs --clean option.

Currently the tools/build-docs was slow
because the clean option was rebulding everything.
But this is only required if one wants the left
sidebar to update.

So now set the default to exclude clean and
add clean option only if --clean is passed.

Also a warning is displayed if clean option is
not passed that the left sidebar won't update.

Fixes #17961.

tools/build-docs

@@ -5,16 +5,30 @@ from lib import sanity_check
 
 sanity_check.check_venv(__file__)
 
+import argparse
 import os
 import sys
 
 ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.append(ZULIP_PATH)
-from scripts.lib.zulip_tools import run
+from scripts.lib.zulip_tools import ENDC, WARNING, run
 
 
 def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("-c", "--clean", action="store_true")
+    args = parser.parse_args()
+
     path = os.path.join(ZULIP_PATH, "docs")
+    if not args.clean:
+        run(["make", "html", "-C", path])
+        print(
+            WARNING
+            + "tools/build-docs --clean is necessary for the navigation/left sidebar to update."
+            + ENDC
+        )
+        return
+
     run(["make", "clean", "html", "-C", path])