docs(core): Add docs for keyboard event binding

[jessicajaniuk]

This adds new documentation on how to bind to keyboard events for key and code fields, and why you might use one over the other.

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Documentation is added to support updates to key event bindings.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information