docs(previews): Add how-to guides, tutorials and an explanation

[ahuseyn]

Description

This PR adds following documentation for the HWP Previews plugin:

• Docs index page
• How-to: Configure HWP Previews
• How-to: Extend HWP Previews
• Explanation: Core-concepts
• Tutorial: Implement Previews with WPGraphQL
• Tutorial: Implement Previews with REST API

Related Issue

• chore: Add docs for HWP Previews #491

Dependant PRs

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?

Screenshots

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

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Latest commit: 92e71eb

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