Add .vercel to gitignore (#4095)

## What are you changing in this pull request and why?

Adds `.vercel` to both `.gitignore` files to prevent Vercel build files
from being included in repo.

_Opened this PR to test if the `deploy/netlify` check was disabled, but
it looks like it still ran. But might as well git this edit in place
while we're at it._

.gitignore

@@ -19,3 +19,6 @@ website/i18n/*
 .netlify
 
 .eslintcache
+
+# Local Vercel folder
+.vercel

website/.gitignore

@@ -26,4 +26,7 @@ yarn-error.log*
 # feeds
 /static/feeds/atom.xml
 /static/feeds/rss.json
-/static/feeds/rss.xml
+/static/feeds/rss.xml
+
+# Local Vercel folder
+.vercel