[ Variables ] - Add missing documentation for repository variables

[sousadiego11]

As seen in this issue the support for variables was made months ago, but the documentation does not have this information

Why update this: at least for me, the only way to discover this functionality existed was searching and finding the mentioned issue above

Reference for the updated section:

[codecov]

Codecov Report

Merging #2032 (b1e5d06) into master (4989f44) will increase coverage by 0.82%.
Report is 246 commits behind head on master.
The diff coverage is 61.96%.

@@            Coverage Diff             @@
##           master    #2032      +/-   ##
==========================================
+ Coverage   61.22%   62.05%   +0.82%     
==========================================
  Files          46       52       +6     
  Lines        7141     8488    +1347     
==========================================
+ Hits         4372     5267     +895     
- Misses       2462     2800     +338     
- Partials      307      421     +114     

Files	Coverage Δ
pkg/common/executor.go	51.69% <100.00%> (+1.69%)	⬆️
pkg/container/docker_cli.go	82.23% <ø> (ø)
pkg/container/docker_logger.go	52.08% <ø> (ø)
pkg/runner/job_executor.go	88.48% <100.00%> (+0.79%)	⬆️
pkg/runner/step_action_local.go	93.54% <100.00%> (ø)
pkg/runner/step_action_remote.go	91.56% <100.00%> (+0.65%)	⬆️
pkg/runner/step_docker.go	93.18% <100.00%> (ø)
pkg/container/file_collector.go	37.30% <0.00%> (ø)
pkg/container/util.go	0.00% <0.00%> (ø)
pkg/container/docker_build.go	60.00% <80.00%> (+1.02%)	⬆️
... and 30 more

... and 2 files with indirect coverage changes

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