docs: add dynamic routes section to dynamic-embeds guide

[goheesheng]

Add a new section explaining Next.js App Router dynamic routes and common pitfalls when building share pages for mini apps.

Key additions:

• Explain how dynamic routes work with [param] syntax
• Warn about multiple dynamic routes at same level causing routing conflicts
• Provide two solutions: type detection or path prefixes
• Include example for dynamic base URL using headers()

This addresses a common issue where developers create conflicting routes like:

• /share/[id]
• /share/[username]
• /share/[wallet]

Which causes unpredictable routing and broken embed previews.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2