Improve test coverage for docs build

Currently the docs build commands in `hashes` and `bitcoin` differ, they
should be the same.

Add a command `cargo doc` to improve coverage e.g., recently we botched
the feature guarding but since CI only runs `cargo rustdoc` with custom
compiler conditional set we didn't catch it.

bitcoin/contrib/test.sh

@@ -80,10 +80,17 @@ cargo run --example ecdsa-psbt --features=bitcoinconsensus
 cargo run --example taproot-psbt --features=rand-std,bitcoinconsensus
 
 # Build the docs if told to (this only works with the nightly toolchain)
-if [ "$DO_DOCS" = true ]; then
+if [ "$DO_DOCSRS" = true ]; then
+    # We use rustdoc so that we can check for broken links.
     RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --all-features -- -D rustdoc::broken-intra-doc-links -D warnings
 fi
 
+# Build the docs with a stable toolchain, in unison with the DO_DOCSRS command
+# above this checks that we feature guarded docs imports correctly.
+if [ "$DO_DOCS" = true ]; then
+    cargo +stable doc --all-features
+fi
+
 # Fuzz if told to
 if [ "$DO_FUZZ" = true ]
 then

hashes/contrib/test.sh

@@ -55,8 +55,15 @@ if [ "$DO_SCHEMARS_TESTS" = true ]; then
 fi
 
 # Build the docs if told to (this only works with the nightly toolchain)
+if [ "$DO_DOCSRS" = true ]; then
+    # We use rustdoc so that we can check for broken links.
+    RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --all-features -- -D rustdoc::broken-intra-doc-links -D warnings
+fi
+
+# Build the docs with a stable toolchain, in unison with the DO_DOCSRS command
+# above this checks that we feature guarded docs imports correctly.
 if [ "$DO_DOCS" = true ]; then
-    RUSTDOCFLAGS="--cfg docsrs" cargo doc --features="$FEATURES"
+    cargo +stable doc --all-features
 fi
 
 # Webassembly stuff