[doc] add tutorial for booster checkpoint

[ver217]

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Closes #3784

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1. Add docstring of Booster about checkpoint
2. Add a tutorial for booster checkpoint
3. Add cuation about ckpt for plugins

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[github-actions]

The code coverage for the changed files is 76%.

Click me to view the complete report

Name                            Stmts   Miss  Cover
---------------------------------------------------
colossalai/booster/booster.py      63     15    76%
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TOTAL                              63     15    76%