Backstage: TechDocs arbitrary file read via mkdocs snippets

Backstage is an open framework for building developer portals. Prior to 1.15.4, the @backstage/plugin-techdocs-node package is affected by techdocs arbitrary file read via mkdocs snippets. Unsafe path resolution in TechDocs source tree handling allows an authenticated user who can register documentation sources to include content from outside the intended documentation boundary. Depending on deployment, this may expose files readable by the build process. This issue is fixed in version 1.15.4.