Specify exact path of dependency license metadata cache in ignore file

The "Check Dependencies" templates store a cache of metadata about the licenses of dependencies under the `.licenses`
folder.

Since this folder only contains generated files, it should be excluded from the validation checks performed by unrelated
tools.

Previously, the markdownlint ignore configuration for this folder did not explicitly specify a location in the root of
the repository, meaning that it would result in ignoring a folder of that name at any location in the project. In cases
like this where a specific path is known, it is best to configure that exact path in order to avoid any chance of
unexpected behavior.

Author: per1234

.markdownlintignore

@@ -1,4 +1,4 @@
 # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/check-markdown/.markdownlintignore
-.licenses/
+/.licenses/
 __pycache__/
 node_modules/