K-API feature: text content refinements

[chrisbc]

• when API mapping fails (S3 Not found error) return a placeholder MD content at {PATH} not available
• provide sample API calls and add to Kororaa UI ticket.

[chrisbc]

query Q_text_content {
  #textual_content {
  #textual_content(index: "help_getting_stopped.md") {
  textual_content(tags: ["glossary", "help"]) {
    ok
    content {
      index
      content_type
      text
      created
      author
      tags
      status
      #text
    }
  }
}