Expand partner documentation about migrating from other solutions to GitLab

To migrate projects from systems other than GitLab, please review the list of [Supported import sources](https://docs.gitlab.com/ee/user/project/import/#supported-import-sources) and [Other Import Sources](https://docs.gitlab.com/ee/user/project/import/#other-import-sources) (anchor link on the same page).

Migrating pipelines from other systems, [like Jenkins](https://docs.gitlab.com/ee/ci/migration/jenkins.html), is a value-added **manual** development process. There are automated tools for such migrations out there, but there's none officially supported by GitLab. We encourage our partners to scope by understanding the number of pipelines, current pipeline performance, [environmental variables](https://docs.gitlab.com/ee/ci/variables/), and secrets used. Partners find a time and materials style contract helpful when consulting on developing pipelines between other source systems and [GitLab's pipeline syntax.](https://docs.gitlab.com/ee/ci/)

Migrating pipelines from other systems is a value-added **manual** development process. There are automated tools for such migrations out there, but there's none officially supported by GitLab. We encourage our partners to scope by understanding the number of pipelines, current pipeline performance, [environmental variables](https://docs.gitlab.com/ee/ci/variables/), and secrets used. Partners find a time and materials style contract helpful when consulting on developing pipelines between other source systems and [GitLab's pipeline syntax.](https://docs.gitlab.com/ee/ci/)

### Jenkins

GitLab Duo Agent Platform's [Convert to GitLab CI/CD Flow](https://docs.gitlab.com/user/duo_agent_platform/flows/foundational_flows/convert_to_gitlab_ci/) can be a great deal of help for these migrations.

### Azure DevOps

Please refer to this extensive [guide on migrating from Azure DevOps to GitLab](https://about.gitlab.com/blog/migration-from-azure-devops-to-gitlab/), authored by GitLab's own Professional Services team.

## From GitLab self-managed to GitLab self-managed