Add a badge for the docs build status which links to the hdoc output

Author: danakj

README.md

@@ -1,4 +1,5 @@
 [![CI](https://github.com/chromium/subspace/actions/workflows/ci.yml/badge.svg)](https://github.com/chromium/subspace/actions/workflows/ci.yml)
+[![docs](https://github.com/chromium/subspace/actions/workflows/docs.yml/badge.svg)](https://docs.hdoc.io/danakj/subspace/)
 # Subspace Library
 
 An experimental take on a safer, simpler C++ standard library.
@@ -7,8 +8,6 @@ Please don't use this library. This is an experiment and we don't yet know where
 it will take us. There will be breaking changes without warning, and there is no
 stable version.
 
-Documentation: https://docs.hdoc.io/danakj/subspace/
-
 # Copyright
 
 All source files must include this header: