[Doc]更清楚的说明ProxySQL的使用方式

[richzhu369]

更清楚的说明ProxySQL的使用方式

Why I'm doing:

当前文档，ProxySQL说要部署在每一台fe中，这个说法不准确，ProxySQL是起到反向代理的作用，最好跟fe服务器分开

What I'm doing:

ProxySQL 应该部署在外部机器，减少fe和observer的压力同时，更能清除的说明，ProxySQL和starRocks是解耦的，也能更加清楚的展示出，ProxySQL和fe之间的反向代理关系。

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.3
  • 3.2
  • 3.1
  • 3.0
  • 2.5

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ DanRoscigno
❌ richzhu369
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[evelynzhaojie]

thanks, we will confirm the content and udpate the doc

[DanRoscigno]

Hi @richzhu369 , please can you sign the CLA?

https://cla-assistant.io/StarRocks/starrocks?pullRequest=43945

[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)