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

crates/docs_preprocessor/Cargo.toml

@@ -1,26 +0,0 @@
-[package]
-name = "docs_preprocessor"
-version = "0.1.0"
-edition = "2021"
-publish = false
-license = "GPL-3.0-or-later"
-
-[dependencies]
-anyhow.workspace = true
-clap.workspace = true
-mdbook = "0.4.40"
-serde.workspace = true
-serde_json.workspace = true
-settings.workspace = true
-regex.workspace = true
-util.workspace = true
-
-[lints]
-workspace = true
-
-[lib]
-path = "src/docs_preprocessor.rs"
-
-[[bin]]
-name = "docs_preprocessor"
-path = "src/main.rs"