Revert "Add docs_preprocessor crate to support Zed Docs" (#16880)

Temporarily revert #16700 to deal with this error:

`error: manifest path `../crates/docs_preprocessor/Cargo.toml` does not
exist` as it was causing the docs-preprocessor not to run, meaning
unexpanded templates were showing up in the public docs.

Reverts zed-industries/zed#16700

Release Notes:

- N/A

Cargo.toml

@@ -24,7 +24,6 @@ members = [
     "crates/db",
     "crates/dev_server_projects",
     "crates/diagnostics",
-    "crates/docs_preprocessor",
     "crates/editor",
     "crates/extension",
     "crates/extension_api",
@@ -166,7 +165,7 @@ members = [
     # Tooling
     #
 
-    "tooling/xtask"
+    "tooling/xtask",
 ]
 default-members = ["crates/zed"]