[INLONG-578][Doc] Add doc for mysql connector for filtering and allmigrate

[EMsnap]

• Fixes [Feature] add doc for mysql connector for filtering and allmigrate #578

Motivation

add doc for mysql connector for filtering and allmigrate

Modifications

add doc for mysql connector for filtering and allmigrate

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[yunqingmoswu]

It is better to add some comments about the whole database migration?