Upgrade FDC3 website to docusaurus >=3.5.2

[kriswest]

The FDC3 website is currently based on docusaurus 2.4.1. NPM audit reports 20 vulnerabilities (14 moderate, 6 high) on that version - the fix for these is to upgrade to Docusaurus 3.5.2, which is a major update with breaking changes, see: https://docusaurus.io/docs/migration/v3

Changes needed will include updates to the documentation markdown as there are differences in how it is handled in v3. Some of these can be applied before upgrading to v3: https://docusaurus.io/blog/preparing-your-site-for-docusaurus-v3