feat: add webhooks plugin documentation

[theodesp]

Description

This PR introduces comprehensive documentation for the WPGraphQL Webhooks plugin. It includes:

• An introductory guide covering the plugin’s purpose, features, and usage.
• Detailed walkthroughs on creating and updating webhooks via the admin UI.
• A new Reference section documenting all available actions and filters with descriptions and example usage.
• A link to the documentation folder added to the main README for easy navigation.

Related Issue

• None directly linked, but addresses the general need for better documentation and developer guidance.

Dependant PRs

• None.

Type of Change

• ✅ Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactoring (no functional changes)
• 📄 Example update (no functional changes)
• 📝 Documentation update
• 🔍 Performance improvement
• 🧪 Test update

How Has This Been Tested?

Documentation changes have been reviewed for clarity, accuracy, and completeness.
The examples and references have been verified against the current plugin codebase to ensure correctness.

Screenshots

N/A

Checklist

• I have read the CONTRIBUTING document
• My code follows the project's coding standards
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• Any dependent changes have been highlighted, merged or published

[github-actions]

ℹ️ Download the latest wp-graphql-headless-webhooks plugin zip from this PR
(See the 'Artifacts' section at the bottom)

[github-actions]

ℹ️ Download the latest hwp-previews plugin zip from this PR
(See the 'Artifacts' section at the bottom)