Protect all blockquotes with norust

[coryan]

We need to parse the CommonMark comments and protect all block quotes with norust.

Background

The Protobuf and OpenAPI v3 documentation comments are in Markdown format, they are supposed to be (aip.dev/192) in CommonMark format, though sometimes they deviate from the AIP.

The generator converts these Protobuf comments into Rustdoc comments.

Rust tries to compile all blockquotes unless protected by a norust mark.

Block quotes appear in many different forms in the CommonMark spec.

In #57 I fixed block quotes protected by backticks (```), then I thought of other ways to specify block quotes.

Overview

We need to parse the CommonMark comments, apply changes in the syntax tree, and re-emit Markdown. Probably in the Codec's FormatDocComments() function. As a starting point, consider this Golang library: https://github.com/yuin/goldmark

Alternatives Considered

We could try to perform this cleanup using heuristics. Shudder. 😨