Add an ability to customize documentation source repo

Documentation deployment script current has the original Ignite repo hardcoded: https://github.com/apache/ignite-website/blob/master/_docs/build.sh#L42

Ignite 3.x is located in a different repo: https://github.com/apache/ignite-3

Suggestion: add a parameter that would allow specifying the version (2.x or 3.x). The script should then pick the corresponding repo URL and fetch the sources from there.