website: add the Back to StackQL Docs link and the source repo link - #3

Changed pages include:

• index.html - view
• 404.html - view
• blog/index.html - view
• contact-us/index.html - view
• install/index.html - view
• providers/index.html - view
• stackql-deploy/index.html - view
• stackqldocs/index.html - view
• tutorials/index.html - view
• blog/product/index.html - view

There are more changes not included in the list