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