README: Briefly document the GitLab CI/CD pipeline and other minor improvements

Describe a bit the current CI/CD setup so we can drop content/designs/image-build-infrastructure.md from the website with no regrets.

This includes:

• 2cd2e988 README: Move the GitLab CI section at the end
• 1b4a9caf README: Use the "GitLab CI/CD" spelling
• bd7e6b49 README: List the imx6qsabrelite u-boot installer image
• 90bd519c README: Briefly document the GitLab CI/CD pipeline

mentioned in merge request docs/apertis-website!78 (merged)

mentioned in merge request docs/apertis-website!77 (merged)

added 6 commits

• 90bd519c...6204764b - 2 commits from branch apertis/v2022dev0
• 1eab8b9c - README: Move the GitLab CI section at the end
• 6fa29161 - README: Use the "GitLab CI/CD" spelling
• f818b846 - README: List the imx6qsabrelite u-boot installer image
• cff763ba - README: Briefly document the GitLab CI/CD pipeline

Compare with previous version

merged