Short answer
HubSpot's KB: export HTML files for pages, blog posts and templates to move content to another platform. Click Actions > Export all pages and blog posts (HTML); it needs Super Admin or Publish permission. Exporting knowledge base articles needs Service Hub Professional or Enterprise, and unsummarized page data needs Marketing or Content Hub Professional or Enterprise.
1. Pages
Website pages in my demo portal.

2. Redirects
Plan redirects before moving. See URL redirects.