datepicker: document new features for 6.0.0

[mmalerba]

• New color option feat(datepicker): add theming support #9407
• New @Outputs feat(datepicker): @Output for year and month selected in multiyear/year #9678
• Icon customization feat(datepicker): allow for mat-datepicker-toggle icon to be customized #9487
• Year selection mode feat(datepicker): add year selection mode #8565

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