Fixes robots.txt to prevent bots from indexing versioned docs

• For https://gitlab.com/gitlab-com/gitlab-docs/issues/195 and https://gitlab.com/gitlab-com/gitlab-docs/issues/221

The versions matched _data/versions.yml:

current:
- '11.0'

previous:
- '10.8'
- '10.7'

offline:
- '10.6'
- '10.5'
- '10.4'
- '10.3'