Add argument to reuse connector docs cache in setup_docs.sh to improve build times

Problem:
The current build process of Flink's documentation involves the `setup_docs.sh` script, which re-clones connector repositories every time the documentation is built. This operation is time-consuming, particularly for developers in regions with slower internet connections or facing network restrictions (like the Great Firewall in China). This results in a build process that can take an excessive amount of time, hindering developer productivity.

Proposal:

We could add a command-line argument (e.g., --use-doc-cache) to the `setup_docs.sh` script, which, when set, skips the cloning step if the connector repositories have already been cloned previously. As a result, developers can opt to use the cache when they do not require the latest versions of the connectors' documentation. This change will reduce build times significantly and improve the developer experience for those working on the documentation.