Add Kurihara n-of-m threshold XOR mnemonic backup (core + tests + docs) - #366

Changed pages include:

• index.html - view
• 404.html - view
• assembly/index.html - view
• build/index.html - view
• communication/index.html - view
• descriptors/index.html - view
• development/index.html - view
• enclosures/index.html - view
• faq/index.html - view
• quickstart/index.html - view

There are more changes not included in the list