change fastmode to fastscan in dev-guide

[joey-yez]

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What is changed, added or deleted? (Required)

Change fastmode to FastScan in dev-guide, and modify TOC

We decided to change the name of the feature fastmode to fast scan.

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• [x] master (the latest development version)
• [x] v6.2 (TiDB 6.2 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 versions)
• [ ] v5.2 (TiDB 5.2 versions)
• [ ] v5.1 (TiDB 5.1 versions)
• [ ] v5.0 (TiDB 5.0 versions)

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• [ ] Delete files
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hfxsd
• ran-huang

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All committers have signed the CLA.

[joey-yez]

/cc @hfxsd please help to review this.

[hfxsd]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5fdeca92e4d92113b6686834db7877296f16db7b

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #9968.