Add 'on_mount' option to menu

[cjodo]

Changes:

• Added the on_mount option to the menu module.
• Updated the relevant documentation to include the new option.

Usage: Users can now pass an on_mount function when creating a menu to execute custom code when the menu is mounted. Example usage:

local Menu = require('nui.menu')

local menu = Menu({
    ...
    on_mount = function()
        print("Menu mounted!")
    end,
    -- other menu options
})