Improve example in Navigation/NavigationView

[Tsuzat]

In a previous update, One can add/remove item(s) in NavigationPane but there was no implementation of this feature in the example application.
So, the following changes are made

• Add the PaneAction named Add New Item which adds a new item in NavigationPane.
• Refactor the code to achieve this, i.e. Moved items outside the Widget Build function and added PaneAction in footerItems.
• Refactor the code snippet according to implementation.

Before

After

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[Tsuzat]

Hey, I have made changes and amended the commit. Also, double quotes have been changes to single quotes in Code Snipppet too.