[ci skip]

Deprecated GitLab CI API clean up

Deprecated GitLab CI API clean up.

The intent here is to clean up deprecated GitLab CI documentation leaving only relevant information.
Since we merged `Ci::Project` to `Project` most of this documentation is outdated.

Closes #13610 

See merge request !3003

add bundle clean to upgrade docs

This MR adds a bundle clean after installing new gems. Bundle clean removes old, not needed gems to keep the system clean.

See merge request !2809

Added Troubleshooting information for most used services.

This replaces the changes started at gitlab-com/www-gitlab-com!1603

I'd like to add similar troubleshooting information for EE only services, but that needs to happen on a different MR on the EE side. Probably after this one gets merged.

cc @ernstvn @dblessing @balameb @cabargas @kelvinmutuma 

See merge request !3080

[ci skip]

[ci skip]

Add support for cross project references for labels

## Summary

Support for cross project references for labels.

## Rationale

1.   Cross project label references are currently not supported in GitLab
1.   `to_reference` method signature in `Label` model breaks the abstraction introduced in `Referable`.

      `concerns/referable.rb:  def to_reference(_from_project = nil)`

      Signatures:

      ```
      label.rb:           def to_reference(format = :id)

      commit_range.rb:    def to_reference(from_project = nil)
      commit.rb:          def to_reference(from_project = nil)
      external_issue.rb:  def to_reference(_from_project = nil)
      group.rb:           def to_reference(_from_project = nil)
      issue.rb:           def to_reference(from_project = nil)
      merge_request.rb:   def to_reference(from_project = nil)
      milestone.rb:       def to_reference(from_project = nil)
      project.rb:         def to_reference(_from_project = nil)
      snippet.rb:         def to_reference(from_project = nil)
      user.rb:            def to_reference(_from_project = nil)
      ```

     This MR suggests using `def to_reference(from_project = nil, format: :id)` which makes use of keyword arguments and preserves abstract interface.

1.   We need support for cross project label references when we want to move issue to another project

     It may happen that issue description, system notes or comments contain reference to label and this reference will be invalid after moving issue to another project and will not be displayed correctly unless we have support for cross project references.

     Merge request that needs this feature: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2831


I think that cross project label references may be useful, (example: `Hey, see our issues for CI in GitLab CE! - gitab-org/gitlab-ce~"CI"`).

cc @JobV @DouweM @rspeicher 

See merge request !2966

Add a TL;DR version in quickstart guide



See merge request !3026