Backdrop clicks on the search dialog were also closing the open mobile nav drawers. Adds an id to <dialog> and references it from both navs' docsClickOutsideIgnore arrays.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• 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?

• 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.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

On mobile, clicking the search dialog backdrop also collapses the open primary and secondary nav drawers — neither nav's click-outside listener recognises the click target as sitting inside the dialog.

What is the new behavior?

Backdrop clicks close only the search dialog; the open mobile nav drawers stay open.

Does this PR introduce a breaking change?

• Yes
• No