docs: improve "Using Pages in FletX" section for clarity and better learning experience

[Beautifulone0]

This PR updates the “Using Pages in FletX” documentation to make it more friendly and beginner-focused.

Changes made:

• Added an engaging introduction explaining what FletXPages are.
• Included simple and progressive examples (Hello Page, Lifecycle Hooks, Controllers, and Counter Page).
• Explained navigation, reactive state, and page utilities clearly.
• Added best practices and a “Next Steps” section with helpful links.
• Improved structure, readability, and pedagogy.

Goal:

Make the documentation more approachable for first-time users and help developers quickly understand how to build and manage pages effectively in FletX.

[Einswilli]

First of all Great job!
The documentation should cover all the features of a FletXPage, including reactive events, listeners, and effects like watch, watch_multiple, etc., performance stats, navigation, and widget builders such as build_app_bar, build_floating_action_button, build_bottom_app_bar, and others.

[Einswilli]

FletXPage must be imported from fletx.core.

[Beautifulone0]

Update: Implemented all reviewer feedback

Changes made:

• Updated import statement now using FletXPage from fletx.core
• Adjusted refresh() method to call super().refresh(_) before reloading data
• Replaced manual increment logic (self.count.value += 1) with self.count.increment()
• Ensured code examples align with the latest FletX API and style conventions