Fix docs build and deploy for releases

[jimthematrix]

The docs are published from the doc-site folder by building with mkdocs and publishing the result to the gh-pages branch, which then acts as the publishing source for Pages. What we want is when a release is published, by pushing tags, the docs site is refreshed with the content. Right now only the head tags is published, but not the release tag. For instance you can see that https://hyperledger-labs.github.io/zeto/head/advanced/locks/ contains the latest docs content, but https://hyperledger-labs.github.io/zeto/latest/ is still at v0.0.11 level.