Add GitLab tab to 'How CircleCI checks out your code' section

[rosieyohannan]

Description

Added a GitLab tab to the "How CircleCI checks out your code" section in the users, organizations, and integrations guide. The new tab explains how CircleCI uses SSH keys to check out GitLab repositories.

Reasons

The GitLab tab was missing from the "How CircleCI checks out your code" section at https://circleci.com/docs/guides/permissions-authentication/users-organizations-and-integrations-guide/#how-circleci-checks-out-your-code. This gap left GitLab users without documentation on how CircleCI checks out their code, while GitHub and Bitbucket users had this information available.

This PR resolves Linear issue DOC-16.

Changes

• Added GitLab tab explaining SSH key creation and usage for code checkout
• Documented how SSH keys are used for checkout operations (main project, submodules, private dependencies, git operations)
• Included reference to GitLab self-managed SSH host key setup
• Fixed initial description to clarify SSH keys are created per project (not per configuration) to match GitHub and Bitbucket tabs

Content checks

Preview your changes:

• View the Vale linter results, select the ci/circleci: lint job at the bottom of your PR. You will be redirected to the vale/lint job output in CircleCI.
• Preview your changes, select the ci/circleci: build job at the bottom of your PR and you will be redirected to CircleCI. Select the Artifacts tab and select index.html to open a preview version of the docs site built for your latest commit.

Content structure:

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Include relevant backlinks to other CircleCI docs/pages.

Formatting:

• Keep the title between 20 and 70 characters.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).

Linear Issue: DOC-16

  

[linear-code]

DOC-16