Support programmatic text in YAML metadata values

[Witiko]

Our current approach in processing metadata always parses values in markdown text. Consider the following line of metadata:

logo: istqb_logo_ctfl

This line is currently parsed as istqblogoctfl. This causes difficult-to-debug errors as discussed with @danopolan today.

Tasks

• During YAML validation in the CI, print a human-readable error when logo metadata values contain _.
• Close Support programmatic text in YAML metadata values Witiko/markdown#440.
• Use jekyllDataProgrammaticString renderers in the definition of the istqb/common/metadata setup snippet.
• Update the Markdown package.
• No longer check for the presence of _ in logo metadata values during YAML validation in the CI.