docs: add tutorial and howto for docs-ng - #32

Changed pages include:

• index.html - view
• 404.html - view
• contributing/index.html - view
• explanation/index.html - view
• how-to/index.html - view
• overview/index.html - view
• reference/index.html - view
• tutorials/index.html - view
• contributing/security.html - view
• contributing/testing/index.html - view

There are more changes not included in the list