Add ddl introduction docs

[TomShawn]

What is changed, added or deleted? (Required)

Add ddl introduction docs

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.4 (TiDB 6.4 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)

What is the related PR or file link(s)?

• This PR is translated from: add document for DDL docs-cn#11440
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hfxsd

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[hfxsd]

/merge

[ti-chi-bot]

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

Commit hash: a56623a

[hfxsd]

/merge

[ti-chi-bot]

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

Commit hash: e75b559

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #12712.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #12713.