docs: rework choosing flavors, add boot-modes and secureboot - #58

Changed pages include:

• index.html - view
• 404.html - view
• contributing/index.html - view
• explanation/index.html - view
• how-to/index.html - view
• introduction/index.html - view
• reference/index.html - view
• tutorials/index.html - view
• contributing/documentation/index.html - view
• contributing/security.html - view

There are more changes not included in the list