Markdown Docs: specify necessary indent (#51456)
Rationale: currently, the Markdown documentation specifies the necessary
indent for code blocks and lists only. As there are people out there,
who indent their lines by only two spaces (or whatever amount)
documenting the indent could help in finding the reason for malformed
Markdown. See #45622. For an example where this problem occurred see
https://github.com/oscar-system/Oscar.jl/pull/1369#discussion_r893488230.
