.gitignore: add doc output
.gitignore: add doc output

ac7b5b2fc8bea87b7a61a3a193671aca707fb820 changed the location of the
docs output but didn't add it to .gitignore.

Do that now.

cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit