[Docs] Updating repository preview info

[fellyph]

This pull request updates the "Quick Start Guide for Developers" documentation to improve clarity, add practical instructions, and expand coverage of previewing pull requests and branches in Playground. The most important changes are grouped below:

Previewing Pull Requests and Branches:

• Added detailed instructions for previewing repository code in Playground using either git:directory or direct .zip URLs, including a JSON Blueprint example and explanation of CI requirements.
• Introduced a new section on previewing WordPress Core and Gutenberg branches or PRs using Query API parameters, with concrete URL examples for trunk and PR previews.

Content and Clarity Improvements:

• Updated the description metadata for the guide to reflect better coverage of embedding WordPress, installing plugins, previewing PRs, and using Playground APIs.
• Reworded plugin and theme installation instructions for clarity and conciseness.
• Revised compatibility testing environment instructions to emphasize configuring PHP and WordPress versions for plugin testing in Playground.

Blueprints API Update:

• Updated the Blueprints API example for importing WXR files to use the file property instead of pluginData for resource URLs.

[fellyph]

use the pull request reference instead branch