Usage docs for root.json file need improvement

Feature Request

Is your feature request related to a problem? Please describe:

#829 already requested improvements in this area, but I think we must do a much better job of explaining the functionality of root.json.

After cloning https://github.com/pingcap/tiup.git and executing make, the behavior of the newly-built tiup executable is extremely confusing.

# ./bin/tiup list
Error: initial repository from mirror(https://tiup-mirrors.pingcap.com/) failed: cannot open the initial root.json at /root/.tiup/bin/root.json

Describe the feature you'd like:

We should do a couple things:

1. Give a clearer explanation in tiup output when root.json cannot be found. There should be an explanation of what this file does and a link to documentation that describes where it comes from and how to get it.
2. Documentation (in docs.pingcap.com) that explains root.json by itself should be created. Today, the only mentions of root.json are in the documentation about creating a mirror, but tiup refuses to do almost anything if that file is missing.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

Start with the behavior exposed by make and bin/tiup list when /root/.tiup/bin/root.json is missing. Review the existing root.json references in the mirror documentation and the docs.pingcap.com structure. Done means the CLI explains the file and how to obtain it, and standalone documentation covers its purpose and source.