refactor!: remove never-implemented SPA adapter remnants and other dead code - #234

Changed pages include:

• index.html - view
• 404.html - view
• adapters/index.html - view
• errors/index.html - view
• examples/index.html - view
• frameworks/index.html - view
• guide/index.html - view
• helpers/index.html - view
• plugins/index.html - view
• adapters/build.html - view

There are more changes not included in the list