Bitbucket Data Center support

[rosieyohannan]

Description

• Updates to onboarding docs with a path for bbdc users
• Add a bitbucket data center integration page
• Update references to bitbucket to specify BB Cloud and Data Center as required
• Add Hello world config sample to sample-config page

See the updates on the docs preview site: https://internal.circleci.com/docs/

• The onboarding flow starts at the first-steps page: https://internal.circleci.com/docs/first-steps/
• Then move to the Create a project page: https://internal.circleci.com/docs/create-project/#create-a-project
• Which then moves you to the Bitbucket Data Center page for integration, pipeline and trigger setup and the CTA to add a config file to your repo: https://internal.circleci.com/docs/bitbucket-data-center-integration/#integrate-a-project-with-circleci

Reasons

BBDC closed preview

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[nbialostosky]

Just a couple comments, but still gonna approve since they are minor nits