Allow clippy::doc_markdown in code-validation and fix lint-tools job

[BD103]

Allow clippy::doc_markdown in code-validation

Fixes #1399!

This change disables the clippy::doc_markdown lint for our code-validation tool. This lint requires code-like text to be surrounded by backticks. The problem with it is that it thinks Tera templates {{ ... }} and front-matter are code-like, and wants backticks around them. Doing so would break the website, though.

While it is nice to catch missing backticks, it's not code-validation's first responsibility. It's first responsibility is to test all of our code blocks to ensure they are valid.

One final note: I originally tried using the [lint] table in Cargo.toml to disable this lint, but workspace = true is incompatible with this: I would have had to duplicate all of [workspace.lints] . Instead, I opted to just use a crate attribute.

Fix lint-tools job

There were several structs and fields that were never used, so I deleted them. Furthermore, IssueError was raising a warning because the fields of its variants were never read. I fixed this by implementing std::error::Error for it, which exposes the source error when the question mark ? operator is used. thiserror was already in our dependency tree, so it should not hurt compile times too much.