Add showcase dir to gitignore.

NOTE: let's fix this madness of copying files in the source tree when running
pnpm run dev.

Author: yuri91

.gitignore

@@ -23,3 +23,4 @@ pnpm-debug.log*
 # content dir copied from astro-theme
 sites/*/src/content/blog
 sites/*/src/content/blogauthors
+sites/*/src/content/showcase