Tolerate markup errors for doc comments (#19607) (#22235)

Follow-up to #21576 (for solving #19607).

1) errors in Markdown mode for `.nim` doc comments are reported with
   red color but allow to generate `.html` with the comment represented by
   literate block (monospaced text). We suppose that it's what people want
   for (supposedly) small doc comments. And this behavior is also a bit
   more Markdown-ish in the sense that Markdown generally does not have
   the concept of parsing error.
   - However, for standalone `.md` it's **not** applied because for large
     files the consequences are way bigger.

(In {.doctype: rst.} mode the behavior is the same as before -- report
the error and stop.)
In future, when our parser can handle Markdown without errors according to
the spec, this code will most probably be not needed.

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