provider/gitlab: Addition of the documentation link for gitlab to sidebar
```
% make testacc TEST=./builtin/providers/gitlab
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/27 05:37:02 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/gitlab -v -timeout 120m
=== RUN TestProvider
--- PASS: TestProvider (0.00s)
=== RUN TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN TestAccGitlabProject_basic
--- PASS: TestAccGitlabProject_basic (41.11s)
=== RUN TestGitlab_validation
--- PASS: TestGitlab_validation (0.00s)
=== RUN TestGitlab_visbilityHelpers
--- PASS: TestGitlab_visbilityHelpers (0.00s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/gitlab 41.125s
```
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