ENV-363 : Add Pagefind search functionality to documentation

[Eklinden]

This PR introduces Pagefind to the documentation to enable full-text search functionality. The search index is generated during the build process and integrated into the site header.

Key changes include:

• Added pagefind as a dependency and updated the build script to generate the search index.
• Integrated the Pagefind UI modal into the header for both desktop and mobile views.
• Configured data-pagefind-body attributes to ensure relevant content is indexed while excluding unwanted sections.
• Added custom styling to match the Envision design system and ensure the search modal respects dark/light mode settings.
• Included a small script to handle modal closing on navigation or link selection.