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.
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.