[ci] add caches for docs jobs

This should speed up docs build times a little, though probably not by
much because we've switched to nightly Rust.

.github/workflows/docs.yml

@@ -16,6 +16,7 @@ jobs:
         with:
           toolchain: nightly
           override: true
+      - uses: Swatinem/rust-cache@c5ed9ba6b7e1bb8aff90d43acd2f0af4990fa57c
       - name: Build rustdoc
         # cargo-compare currently pulls in cargo which bloats build times massively
         run: |