[doc] setup new doc build service

[Rhett-Ying]

🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

Description

Due to security issue, I've upgraded the OS of doc build server(It's based on https://about.readthedocs.com/) from ubuntu18.04 to ubuntu20.04. After upgrade, some packages are missing. So the doc build server is down: http://doc-build.dgl.ai/projects/dgl/builds/.

This is a good chance to setup a new doc build server with following improvements:

1. we hard coded here and there in previous doc build server.
2. mxnet/TF could be deprecated.
3. If it's not easy to maintain multiple previous versions, we could skip some of them. just keep the latest and stable versions only.

Depending work items or issues