Docs(Plugins): Refactor plugins item to be a submenu of Scheduler

[JesseStutler]

Goal

Refactor the "Plugins" to be a submenu of the "Scheduler" menu in the website's navigation.

Context

Plugin is now an item in Schedulers and is a document. It is now expected that the plugin can be a sub-menu, and when the sub-menu is dropped down, it can display the documents of different plugins, such as the gang plugin, predicate plugin, etc. Since the Hugo version is quite old, 0.57.2, some CSS and JS code changes are needed.

Acceptance Criteria

• The website's navigation is updated to show "Plugins" as a submenu under "Scheduler", when click the plugins item, it can be dropped down and show the docs of different plugins.
• The site builds successfully without any errors related to this change.

[JesseStutler]

/help

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[JesseStutler]

Will let Copilot do it first, but still welcome contributors if you can handle it if copilot cannot handle

[hajnalmt]

/cc