Add markdown version of migration guide

[dturner]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @dturner, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the Navigation 3 migration guide by converting it to a more standardized markdown format, enhancing its utility for AI agents, and providing comprehensive code examples for implementing Navigation 3's state management. The changes aim to clarify the migration process from Navigation 2 to Navigation 3, particularly concerning the replacement of NavController with Navigator and NavigationState, and the use of NavDisplay.

Highlights

• Markdown Formatting: Removed explicit anchor IDs from section headings (e.g., {:#use-ai-agent}) for cleaner markdown syntax.
• AI Agent Integration: Added a note indicating that AI agent usage with the guide is experimental, along with a link to file issues. An 'AI Agent' instruction was also added to prompt assumption checks.
• Navigation State Management: Introduced detailed Kotlin code snippets for NavigationState.kt and Navigator.kt, providing concrete examples for managing navigation state in Navigation 3.
• Component Mapping: Updated tables and textual descriptions to clearly map NavController methods and fields to their Navigator and NavigationState equivalents.
• NavHost Replacement: Refined instructions for replacing NavHost with NavDisplay, including guidance on using navigationState.toEntries(entryProvider).
• Documentation Clarity: Made several minor textual adjustments throughout the guide for improved clarity and conciseness, and standardized external links by removing {:.external}.

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[gemini-code-assist]

Code Review

This pull request updates the migration guide from Navigation 2 to Navigation 3. The changes include adding detailed code snippets for NavigationState and Navigator, clarifying several steps, fixing typos, and improving the markdown formatting by using semantic table headers and removing processor-specific syntax. The guide is now more comprehensive and easier to follow. I've suggested a couple of improvements to the new code snippets to fix a bug and make the code more idiomatic.

[gemini-code-assist]

The implementation of the stacksInUse computed property can be simplified to be more concise and idiomatic. Using an if/else expression is cleaner than creating and modifying a mutable list.

Suggested change

	val stacksInUse: List<NavKey>
	get(){
	val stacksInUse = mutableListOf(startRoute)
	if (topLevelRoute != startRoute) stacksInUse += topLevelRoute
	return stacksInUse
	}
	}
	val stacksInUse: List<NavKey>
	get() = if (topLevelRoute == startRoute) {
	listOf(startRoute)
	} else {
	listOf(startRoute, topLevelRoute)
	}

[gemini-code-assist]

The string template in the error message is incorrect. It should use curly braces ${...} to correctly interpolate the topLevelRoute property of the state object. Without them, it will call toString() on the state object and append the literal string .topLevelRoute, which can be misleading when debugging.

Suggested change

	val currentStack = state.backStacks[state.topLevelRoute] ?:
	error("Stack for $state.topLevelRoute not found")
	val currentStack = state.backStacks[state.topLevelRoute] ?:
	error("Stack for ${state.topLevelRoute} not found")