[CELEBORN-1678][FOLLOWUP] Improve Celeborn CLI user guide

[SteNicholas]

What changes were proposed in this pull request?

Improve Celeborn CLI user guide including:

• Add license of Celeborn CLI user guide.
• Optimize the introduction of setup and usage for Celeborn CLI.
• Optimize the navigation of Celeborn CLI to combine Celeborn Ratis Shell.

Why are the changes needed?

There is no license in Celeborn CLI user guide. Meanwhile, there are certain improvement in user guide including the license, navigation, and the introduction of setup and usage.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

No.

[SteNicholas]

Ping @akpatnam25, @RexXiong, @FMX.