Docs: Update documentation with CI variable types usage example

See merge request gitlab-org/gitlab-ce!28515

(cherry picked from commit 0702d4b6)

706dfdb7 Add CI variable types usage example
7b9a52c0 Update screenshot to match latest CI variables UI
83cd1a76 Fix typo in CI variables docs
3e5b4ce8 Apply suggestion to doc/ci/variables/README.md
b7d820a4 Apply suggestion to doc/ci/variables/README.md
9d2bef55 Apply suggestion to doc/ci/variables/README.md
cb637cc9 Apply suggestion to doc/ci/variables/README.md
c13648a8 Apply suggestion to doc/ci/variables/README.md
121917cb Apply suggestion to doc/ci/variables/README.md
cf95e5ec Apply suggestion to doc/ci/variables/README.md
556d5ebf Apply suggestion to doc/ci/variables/README.md