fix (printing docs): Added printer friendly code-example background color rule

[SetTrend]

The original SCSS file for printing the documentation is missing a background-color rule for printing, so color/background-color happen to be the same.

I added a light grey background color rule that saves toner and still is immediately recognizable as distinct code example token to the reader.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Currently, CSS color/background-color on the printed version of code examples in the documentation happen to be the same, so the code examples are plain invisible to the reader.
Issue Number: #23431

What is the new behavior?

I added a light grey background color rule to the SCSS file for printing that saves toner and still is immediately recognizable as distinct code example token to the reader:

Does this PR introduce a breaking change?

[ ] Yes
[X] No

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