ci(deploy_docs.sh): generalize for use in doc-gen CI (#2978)
This moves some installation steps out of `deploy_docs.sh` script and makes it accept several path arguments so that it can be re-used in the CI for `doc-gen`.
The associated `doc-gen` PR: https://github.com/leanprover-community/doc-gen/pull/27 will be updated after this is merged.
