docs: add guide on working with concepts API - #137

Changed pages include:

• index.html - view
• 404.html - view
• api/index.html - view
• concepts/index.html - view
• licensing/index.html - view
• search/index.html - view
• self-hosting/index.html - view
• troubleshooting/index.html - view
• faq/faqs/index.html - view
• faq/git/index.html - view

There are more changes not included in the list