Remove docs badge

Docs.rs won't build documentation because the crate is a bin.

The badge might return later if I decide to make it a library.

README.md

@@ -1,5 +1,4 @@
 [![Crates.io](https://img.shields.io/crates/v/hd)](https://crates.io/crates/hd "Crates.io version")
-[![Documentation](https://img.shields.io/docsrs/hd)](https://docs.rs/hd "Documentation")
 [![unsafe forbidden](https://img.shields.io/badge/unsafe-forbidden-success.svg)](https://github.com/rust-secure-code/safety-dance/)
 [![GitHub actions](https://img.shields.io/github/actions/workflow/status/parasyte/hd/ci.yml?branch=main)](https://github.com/parasyte/hd/actions "CI")
 [![GitHub activity](https://img.shields.io/github/last-commit/parasyte/hd)](https://github.com/parasyte/hd/commits "Commit activity")