docs(templates): Fix broken API documentation anchor links

What does this MR do and why?

Updates outdated GitLab API documentation anchor links in the templates package (ci_yml_templates.go, dockerfile_templates.go, gitignore_templates.go) to match the current headings on docs.gitlab.com:

• ci_yml_templates.go:
  • #list-all-cicd-templates
  • #retrieve-details-of-a-cicd-template
• dockerfile_templates.go:
  • #list-all-dockerfile-templates
  • #retrieve-a-single-dockerfile-template
• gitignore_templates.go:
  • #list-all-gitignore-templates
  • #retrieve-a-single-gitignore-template

Related issues: None open.