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/templates.rs

@@ -0,0 +1,25 @@
+use crate::PreprocessorContext;
+use regex::Regex;
+use std::collections::HashMap;
+
+mod action;
+mod keybinding;
+
+pub use action::*;
+pub use keybinding::*;
+
+pub trait Template {
+    fn key(&self) -> &'static str;
+    fn regex(&self) -> Regex;
+    fn parse_args(&self, args: &str) -> HashMap<String, String>;
+    fn render(&self, context: &PreprocessorContext, args: &HashMap<String, String>) -> String;
+
+    fn process(&self, context: &PreprocessorContext, content: &str) -> String {
+        self.regex()
+            .replace_all(content, |caps: &regex::Captures| {
+                let args = self.parse_args(&caps[1]);
+                self.render(context, &args)
+            })
+            .into_owned()
+    }
+}