Fix up `grep` pattern

Change from `^/site` to `^site/`
Fix `grep` pattern in `script/test` as well
Merge pull request #2608 from alfredxing/fix-proof-grep
Update history to reflect merge of #2608 [ci skip]
We need to use `bundle exec jekyll` in `script/proof`
Add `set -e` to top of `proof` and `cibuild`

Exit immediately on any non-zero exit code
Merge pull request #2610 from alfredxing/proof-use-bundle-exec
Update history to reflect merge of #2610 [ci skip]
Add not available link
Add available link
Add second not available link