Build script: improved accessibility

[ralfhandl]

Builds on

• Build script: no local biblio, use only Specref ids #3992
• Build script: improved code blocks #4006

Fixes #3946

• bibliography references underlined
• code blocks with tabindex="0"
• contrast ratio of green headlines
• formatting of headlines
• contrast ratio of code blocks: almost everything is too low

Compare

• https://ralfhandl.github.io/OpenAPI-Specification/oas/latest-a11y.html
  
• https://spec.openapis.org/oas/latest.html
  

[handrews]

@stringtheory

[lornajane]

As a user with some accessibility needs, I really appreciate that the left sidebar doesn't move around in this version, it's just part of the page! I always have to sort of drill into the bit of OpenAPI spec I want by skipping into the fixed fields, and trying to navigate from there since I can't scroll the left bar.