[doc] add tutorial for booster checkpoint (#3785)

* [doc] add checkpoint related docstr for booster

* [doc] add en checkpoint doc

* [doc] add zh checkpoint doc

* [doc] add booster checkpoint doc in sidebar

* [doc] add cuation about ckpt for plugins

* [doc] add doctest placeholder

* [doc] add doctest placeholder

* [doc] add doctest placeholder

docs/source/en/basics/booster_checkpoint.md

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+# Booster Checkpoint
+
+Author: [Hongxin Liu](https://github.com/ver217)
+
+**Prerequisite:**
+- [Booster API](./booster_api.md)
+
+## Introduction
+
+We've introduced the [Booster API](./booster_api.md) in the previous tutorial. In this tutorial, we will introduce how to save and load checkpoints using booster.
+
+## Model Checkpoint
+
+{{ autodoc:colossalai.booster.Booster.save_model }}
+
+Model must be boosted by `colossalai.booster.Booster` before saving. `checkpoint` is the path to saved checkpoint. It can be a file, if `shard=False`. Otherwise, it should be a directory. If `shard=True`, the checkpoint will be saved in a sharded way. This is useful when the checkpoint is too large to be saved in a single file. Our sharded checkpoint format is compatible with [huggingface/transformers](https://github.com/huggingface/transformers).
+
+{{ autodoc:colossalai.booster.Booster.load_model }}
+
+Model must be boosted by `colossalai.booster.Booster` before loading. It will detect the checkpoint format automatically, and load in corresponding way.
+
+## Optimizer Checkpoint
+
+> ⚠ Saving optimizer checkpoint in a sharded way is not supported yet.
+
+{{ autodoc:colossalai.booster.Booster.save_optimizer }}
+
+Optimizer must be boosted by `colossalai.booster.Booster` before saving.
+
+{{ autodoc:colossalai.booster.Booster.load_optimizer }}
+
+Optimizer must be boosted by `colossalai.booster.Booster` before loading.
+
+## LR Scheduler Checkpoint
+
+{{ autodoc:colossalai.booster.Booster.save_lr_scheduler }}
+
+LR scheduler must be boosted by `colossalai.booster.Booster` before saving. `checkpoint` is the local path to checkpoint file.
+
+{{ autodoc:colossalai.booster.Booster.load_lr_scheduler }}
+
+LR scheduler must be boosted by `colossalai.booster.Booster` before loading. `checkpoint` is the local path to checkpoint file.
+
+## Checkpoint design
+
+More details about checkpoint design can be found in our discussion [A Unified Checkpoint System Design](https://github.com/hpcaitech/ColossalAI/discussions/3339).
+
+<!-- doc-test-command: echo  -->