Build script: improved code blocks

[ralfhandl]

Builds on

• Build script: no local biblio, use only Specref ids #3992

• code blocks are sequentially focusable (for example via TAB key)
• no empty line at the start of each code block

Please compare

• with this improvement: https://ralfhandl.github.io/OpenAPI-Specification/oas/latest-code.html#format
• current state: https://ralfhandl.github.io/OpenAPI-Specification/oas/latest.html#format

[rvedotrc]

Could you perhaps change the base of this PR, to the head of the previous PR?