Don't rely on relative path for docs preprocessor (#16883)

Reapplies #16700 with a corrected command. Now it no longer relies on a
relative path.

Thanks @maxdeviant for the quick help 🙏 

Release Notes:

- N/A

crates/docs_preprocessor/src/main.rs

@@ -0,0 +1,58 @@
+use anyhow::{Context, Result};
+use clap::{Arg, ArgMatches, Command};
+use docs_preprocessor::ZedDocsPreprocessor;
+use mdbook::preprocess::{CmdPreprocessor, Preprocessor};
+use std::io::{self, Read};
+use std::process;
+
+pub fn make_app() -> Command {
+    Command::new("zed-docs-preprocessor")
+        .about("Preprocesses Zed Docs content to provide rich action & keybinding support and more")
+        .subcommand(
+            Command::new("supports")
+                .arg(Arg::new("renderer").required(true))
+                .about("Check whether a renderer is supported by this preprocessor"),
+        )
+}
+
+fn main() -> Result<()> {
+    let matches = make_app().get_matches();
+
+    let preprocessor =
+        ZedDocsPreprocessor::new().context("Failed to create ZedDocsPreprocessor")?;
+
+    if let Some(sub_args) = matches.subcommand_matches("supports") {
+        handle_supports(&preprocessor, sub_args);
+    } else {
+        handle_preprocessing(&preprocessor)?;
+    }
+
+    Ok(())
+}
+
+fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<()> {
+    let mut stdin = io::stdin();
+    let mut input = String::new();
+    stdin.read_to_string(&mut input)?;
+
+    let (ctx, book) = CmdPreprocessor::parse_input(input.as_bytes())?;
+
+    let processed_book = pre.run(&ctx, book)?;
+
+    serde_json::to_writer(io::stdout(), &processed_book)?;
+
+    Ok(())
+}
+
+fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
+    let renderer = sub_args
+        .get_one::<String>("renderer")
+        .expect("Required argument");
+    let supported = pre.supports_renderer(renderer);
+
+    if supported {
+        process::exit(0);
+    } else {
+        process::exit(1);
+    }
+}