docs: simplify example headers by showing only the file name

[MeAkib]

Removed redundant src/app/ prefixes from headers to make examples cleaner and easier to read. The full path remains available via the path attribute, so the shortened headers improve clarity without losing context.

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-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)
• [x] 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?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[JeanMeche]

Please revert the changes in reference and tools/cli

We still have a change in tools/cli

[AndrewKushnir]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 63538524bcf255ac1d7a46264b339d695abcdb0a
• 21.0.x: 4d3b2bff88e6d24dcc2330d4d12c09770268e565