Restructure CI/CD Admin settings documentation

What does this MR do and why?

This MR reorganizes the CI/CD Admin area settings documentation to better match the UI structure.

This MR is part 1 of splitting the larger changes from !188611 (closed) into more manageable reviews:

• Updates the page title and introduction.
• Reorganizes existing content to group related settings.
• Improves navigation and findability of settings.

Key changes

• Updated the page title from "CI/CD Admin area settings" to "CI/CD settings".
• Updated the introductory section listing all available settings categories.
• Restructured the documentation to align with the UI, with separate sections for:
  • Continuous integration and deployment
  • Package registry
  • Runners
  • Job token permissions
  • Job logs
• Reorganized content within the Continuous Integration and Deployment section to group related settings:
  • Auto DevOps
  • Instance runners
  • Job artifacts
  • Jobs
  • Pipelines
  • CI/CD limits

What's NOT included in this MR:

• No changes to the actual topics.
• Content updates will come in subsequent MRs.

Review app link

https://docs.gitlab.com/upstream-review-mr-ee-188635/administration/settings/continuous_integration/

References

• !188611 (closed)