chore: pin action references in README examples to commit SHA

Replace non-existent @v1 tag with SHA-pinned reference to the
latest release v0.2.3 in both usage examples.

Co-Authored-By: OpenCode <noreply@opencode.ai>

Author: 0x46616c6b

README.md

@@ -35,7 +35,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: Setup Node
-        uses: Staffbase/backstage-techdocs-action@v1
+        uses: Staffbase/backstage-techdocs-action@892be7cc155022260e34cf0330f8424946b55c11 # v0.2.3
         with:
           entity-name: 'pizza-service'
           publisher-type: 'azureBlobStorage'
@@ -64,7 +64,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: Setup Node
-        uses: Staffbase/backstage-techdocs-action@v1
+        uses: Staffbase/backstage-techdocs-action@892be7cc155022260e34cf0330f8424946b55c11 # v0.2.3
         with:
           entity-name: 'pizza-service'
           additional-plugins: 'mkdocs-minify-plugin\>=0.3 mkdocs-awesome-pages-plugin==2.8.0 mdx_include==1.4.2'