docs: Rewrite tools/build-docs in python.

Change the script to python. This is done
for the following reasons.
* It enables us to use the sanity_check.
* Later when we add warning to include
--clean flag we can use the pre-existing
WARNING from zulip_tools rather than using
terminal color codes.

TODO: Currently this script is slow as the
clean option is expensive so instead use only
html by default and clean only if --clean
option is passed.

Part of #17961.

tools/build-docs

@@ -1,3 +1,22 @@
-#!/usr/bin/env bash
-set -eu
-make -C "$(dirname "$0")"/../docs clean html
+#!/usr/bin/env python3
+
+# check for the venv
+from lib import sanity_check
+
+sanity_check.check_venv(__file__)
+
+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
+
+
+def main() -> None:
+    path = os.path.join(ZULIP_PATH, "docs")
+    run(["make", "clean", "html", "-C", path])
+
+
+if __name__ == "__main__":
+    main()