Changes in FDP primer

• Minor grammar tweaks
• Improvements in AsciiDoc syntax
• Improvements in css code blocks
• Display language on hover in code blocks
• Use console instead shell in some code blocks[1]
• Add link for Flycheck

PR: 268764

[1] https://docs.asciidoctor.org/asciidoc/latest/verbatim/source-highlighter/#shell-vs-console