Add zip_docs

[rwinch]

rsync-antora-reference should create a zip file that includes all the documentation at the location of ${httpdocs-path}/${github-repository-name}-docs.zip. The value of ${github-repository-name} is the name portion of the GitHub repository in the form of ${owner}/${name}.

Related: #34

[madorb]

is there a link in the main documentation to the zip files? I don't see one, and i must be doing something wrong as

https://docs.spring.io/spring-security/reference/spring-security-docs.zip works but
https://docs.spring.io/spring-boot/reference/spring-boot-docs.zip does not