Update parallelization page with ci build id and specs explanation

[bahmutov]

Part 1

In https://docs.cypress.io/guides/guides/parallelization.html#CI-Build-ID-environment-variables-by-provider under environment variables we need to add example how to override the default value. For example for Jenkins we use BUILD_NUMBER but some people might think BUILD_TAG is better, so we should show

cypress run --record --parallel --ci-build-id $BUILD_TAG

There is a link with a single mention of ci-build-id above

Part 2

A user has asked how the dashboard gets the list of specs to load balance - and I had to explain that the test runner is sending them on the start. I don't see it explained in the page, so we should mention that the first thing the test runner does is sending them to the service.

[bahmutov]

For second part, the confusion is from the way the first steps is described in

I mean, what is "CI machines contact the Cypress Dashboard Service to indicate which spec files to run in the project."

[kusumkappdirect]

Can cypress parallelization work with argo-cd ?