[FLINK-22171][sql-client][docs] Update the SQL Client doc

[fsk119]

What is the purpose of the change

Update the SQL Client doc.

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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[wuchong]

Please update the top sql client picture link to following, the picture is not found in current website.

{{< img src="/fig/sql_client_demo.gif" alt="Animated demo of the Flink SQL Client CLI running table programs on a cluster" width="80%" height="80%" >}}

Besides, I think we can improve the TOC a bit more, what do you think about the following TOC:

Getting Started
 - Starting the SQL Client CLI
 - Running SQL Queries
Configuration
 - SQL Client startup options
 - SQL Client Configuraiton
 - Initialize session using SQL file
 - Dependencies
Submitting SQL Jobs
 - Interactive Command Line
 - Execute SQL Files
 - Execute a set of SQL statements
 - Execute DML statements sync/async
 - Start a SQL Job from a savepoint
 - Define a Custom Job Name
Compatibility
Limitations & Future

In Start a SQL Job from a savepoint section, would be better to also mention "You can use Flink command line to create and manage savepoints, see more Creating a Savepoint".

[wuchong]

I left some final comments.

[wuchong]

LGMT.

[wuchong]

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