iAPI Docs: Add client-side navigation compatibility guide

[DAreRodz]

What?

Adds a new documentation page: Client-Side Navigation Compatibility Guide for the Interactivity API.

Why?

Block developers and theme authors need clear guidance on what makes their code compatible with client-side navigation. Without this, developers may unknowingly introduce patterns that break client-side navigation — causing fallbacks to full page reloads or potentially breaking the site, degrading the user experience.

There was no centralized reference for CSN compatibility, nor a checklist that developers could use to verify compatibility before declaring their blocks compatible.

How?

Adds a new document at docs/reference-guides/interactivity-api/core-concepts/client-side-navigation-compatibility.md and links it from the Core Concepts README. The guide covers:

• Declaring compatibility in block.json via supports.interactivity.clientNavigation.
• What makes a block compatible — non-interactive blocks, Interactivity API blocks, and blocks using other libraries.
• CSS requirements — no dynamic CSS injection, no runtime stylesheet modifications, stable CSS selectors (with wp_unique_id_from_values() as the recommended alternative to wp_unique_id()), and selector consistency across navigations.
• JavaScript requirements — use of script modules instead of regular scripts, and how to handle script modules outside blocks (e.g., classic PHP themes) via wp_register_script_module() with loadOnClientNavigation.
• HTML requirements — consistent HTML structures, proper use of data-wp-key for dynamic lists, avoiding DOM mutations outside the Interactivity API (with data-wp-watch as the escape hatch), and using attachTo for HTML outside router regions.
• Compatibility checklist — a pre-flight checklist for developers to verify before marking a block as CSN-compatible.
• Quick reference table summarizing block types and their compatibility status.

The guide is written to be useful for both block developers and classic PHP theme/plugin authors.