docs: Remove unneeded ignored directories

npm run doc outputs generated files in docs/frontend/ directory
rather than documentation/frontend/dist. All generated files under
docs/frontend are already ignored in docs/.gitignore.

Change-Id: I2b9db6ba7885edd7e9ce0c748472c22afdc729ce

docs/.gitignore

@@ -2,4 +2,3 @@
 # .gitignore can't be placed inside docs/frontend because VitePress build
 # empties the directory when building.
 frontend/*
-frontend/!.gitignore

documentation/frontend/.gitignore

@@ -1,3 +1,2 @@
 node_modules/
 docs/.vitepress/cache
-dist/