docs(material/input): add example for showing errors from a parent form group

[RAJEEV2510]

Fixes #8513

Errors from a validator on a parent FormGroup, such as a check that two fields match, are set on the group rather than on the input. The input's own control stays valid, so <mat-error> never shows them by default. This comes up often (see the issue thread), and the docs don't cover it.

• Adds an input-parent-form-errors example: a password and confirm-password form with a group-level passwordsMismatch validator, and an ErrorStateMatcher that shows that group error on the confirm field once it has been interacted with or the form is submitted.
• Adds a short paragraph and the example to input.md under "Changing when error messages are shown", right after the existing input-error-state-matcher example.
• Registers the example in src/components-examples/material/input/index.ts.

Docs-only; no library code changes.

Verification

• bazel build //src/components-examples/material/input:input //src/components-examples:docs-content succeeds, and the example appears in the generated docs content.
• ng-dev format, tslint and stylelint pass on the changed files.
• The matcher and form from the example behave as intended: no error while pristine, the mismatch is shown after the confirm field is dirty or the form is submitted, it clears once the passwords match, and the required error still shows.