ci: Make docs-only PRs only trigger docs-related tests (#18744)

This should speed up any docs-only PRs so that they don't have to run the full 5 minute battery of tests.

Release Notes:

- N/A

script/check-spelling

@@ -1,6 +1,9 @@
 #!/bin/sh
 
+set -eu
+
 TYPOS_CLI_VERSION=1.24.6
+TARGET_DIR=${1:-""}
 
 if ! cargo install --list | grep "typos-cli v$TYPOS_CLI_VERSION" > /dev/null; then
     echo "Installing typos-cli@$TYPOS_CLI_VERSION..."
@@ -8,4 +11,4 @@ if ! cargo install --list | grep "typos-cli v$TYPOS_CLI_VERSION" > /dev/null; th
 else
     echo "typos-cli@$TYPOS_CLI_VERSION is already installed."
 fi
-typos
+typos $TARGET_DIR