Generate user cli doc

[Xenorith]

• Add a new CLI command that iterates through the command tree and generates a markdown file based on each command's definition and description
• Migrate all the content in the previous User-CLI.md into the corresponding commands in golang code, mainly updating their Long and Examples fields
• Run bin/alluxio generate user-cli to write the generated content directly into docs/en/operation/User-CLI.md

[Xenorith]

@Zijian-Zhu the main content to review is in the first commit: 3b7604d

all other changes are to migrate the content of User-CLI.md to their respective golang commands.
note that the User-CLI.md file is now 100% generated, similar to the csv and yaml files in docs/_data/generated/. but since the generated content is in markdown, i could not follow the pattern of generating the file in the generated/ directory and embed the contents.

this file also faces the similar maintenance problem as the generated files. if someone changes the CLI commands, it will cause the User-CLI.md file to be out of date. someone needs to manually run the bin/alluxio generate user-cli command to regenerate the file and commit the diff.

[Xenorith]

alluxio-bot, merge this please