Add Checkmark marker type to WPF release and marker documentation

[Copilot]

This updates the WPF docs for the newly introduced Checkmark chart marker in v26.1. It adds the release-note coverage requested for both English and Japanese content and extends the existing marker template references to include the new enum value.

• Release notes

  • Added a new What's New in 2026 Volume 1 topic in:
    • topics/en/whats-new-in-2026-volume-1.adoc
    • topics/ja/whats-new-in-2026-volume-1.ja-JP.adoc
  • Documented Checkmark as a new marker shape available for chart series markers and legend badges.

• Changelog

  • Added a new 26.1 entry in:
    • topics/en/changelog.adoc
    • topics/ja/changelog.ja-JP.adoc
  • Captured the Checkmark marker type under chart enhancements.

• Revision history

  • Linked the new 2026 Volume 1 topic from:
    • topics/en/wpf-revision-history.adoc
    • topics/ja/wpf-revision-history.ja-JP.adoc

• Existing marker references

  • Updated the historical marker template list to include Checkmark in:
    • topics/en/breaking-changes-in-2016-volume-2.adoc
    • topics/ja/breaking-changes-in-2016-volume-2.ja-JP.adoc

=== Checkmark Marker Type

Charts now support a new `Checkmark` marker type that can be used for series markers and legend badges.

This marker option is available anywhere the `MarkerType` and `LegendItemBadgeShape` enumerations are used for chart marker shapes.