[Doc] add extensions docs

[stdpain]

Why I'm doing:

What I'm doing:

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 pr needs auto generate documentation
• 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
  • 4.0
  • 3.5
  • 3.4
  • 3.3

---

Note

Adds a new doc page describing FE static extensions, including build/setup, entrypoint registration, deployment location, and startup logs.

• New docs/en/sql-reference/sql-statements/cluster-management/extension/extensions.md explains what static extensions are and how they differ from dynamic plugins
• Provides Maven maven-jar-plugin config to build a separate -ext.jar
• Shows extension entrypoint (@SRModule, onLoad, ctx.register(...)) example
• Details deployment steps (place *-ext.jar in Config.ext_dir) and sample FE startup log messages

^{Written by Cursor Bugbot for commit 8e0d387. This will update automatically on new commits. Configure here.}

[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%)

[alvin-celerdata]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!