ticdc: add docs about ticdc open api

[asddongmen]

What is changed, added or deleted? (Required)

Add docs about the new Open APIs of TiCDC.

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 (in Chinese).

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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

• This PR is translated from:
• Other reference link(s):

Develop openapi for ticdc. #2411
*: add some open API to cdc. #2416
http_*: add create and update changefeed API #2472

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

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[asddongmen]

@hi-rustin @amyangfei @leoppro @ben1009 please help to review this PR. Thanks~!!!!

[asddongmen]

This pr is updated in #6846