Switch out GCS for PageFind on subsites

[cjyabraham]

GCS doesn't seem to produce very good search results on small sites. It'd be better to remove our reliance on Google as well. Instead we can use PageFind, which is an open source local search for static sites.

Current issues with GCS on the subsites:

• https://github.com/cncf/glossary/issues/755 addressed by PageFind multilingual support
• https://github.com/cncf/glossary/issues/2087
• It's not open source
• It produces poor or no results on the smaller sites

Subsites:

• [x] Glossary PR
• [x] OSPO Book PR
• [x] OSPO Glossary PR
• [x] TAG Runtime PR
• [x] TAG Observability PR
• [x] TAG Env Sustainability PR
• [x] TAG App Delivery PR
• [x] Maturity Model PR
• [x] Contributors PR
• [x] TAG Security PR