docs: Fix and add docs for Level

[icecream17]

Have the documentation for Level reflect actual appearance.

---

The output from this file:

(43 lines)

//! Provides a user-friendly error if the given file has the string `tab`
//!
//! A silly utility to test annotate_snippets (and anstream)

use annotate_snippets::{Level, Renderer, Snippet};
use clap::Parser;

#[derive(Parser)]
struct Cli {
    file_path: std::path::PathBuf,
}

fn main() {
    let Cli { file_path } = Cli::parse();
    let path_string = file_path.to_string_lossy().to_string();

    // Bad error message; don't do this!
    let contents =
        std::fs::read_to_string(file_path).expect("Should have been able to read the file");

    let renderer = Renderer::styled();
    let message = if let Some(index) = contents.find("tab") {
        Level::Error.title("no tabs allowed!").snippet(
            Snippet::source(&contents)
                .line_start(1)
                .origin(&path_string)
                .fold(true)
                .annotation(
                    Level::Error
                        .span(index..(index + 3))
                        .label("Found `tab` here"),
                )
                .annotation(Level::Warning.span(index..(index + 3)).label("Warning"))
                .annotation(Level::Info.span(index..(index + 3)).label("Info"))
                .annotation(Level::Note.span(index..(index + 3)).label("Note"))
                .annotation(Level::Help.span(index..(index + 3)).label("remove `tab`")),
        )
    } else {
        Level::Info.title("No \"tabs\" found, congrats!")
    };
    anstream::println!("{}", renderer.render(message));
}

Results in this output:

[icecream17]

Note: this is potentially inconsistent with cargo and clippy

[icecream17]

Note: This is cyan rather than blue:
(found by using manual output while trying to work around #101:)

If a reviewer comments, I will change the wording to "cyan"

[Muscraft]

Thank you for wanting to make them consistent. However, I think the original doc comments mentioning the color and character was not the right call. This is primarily due to three reasons:

1. Colors are configurable by end users
2. Colors are platform dependent
3. Characters being tied to a Level is for historical reasons and will be removed in the future when we match rustc's idea of Primary and Secondary for Annotations

I do think it is a good idea to document the Level text and default color, but graphically and in a way that won't go stale. This can be done by utilizing fixture tests and including the SVGs as doc comments. The rough process would be:

• Add a new docs folder inside of tests/fixtures for the new tests
• Add a basic example for each Level
• Link to the newly created SVGs in the same way as we do in the lib docs