Document new WPF chart MarkerSize property across product and release topics

[Copilot]

Adds documentation for the newly introduced chart-series MarkerSize property and propagates it through the current release documentation. This updates both English and Japanese content, including the release changelog and 2026 Volume 1 What’s New coverage.

• Product docs

  • Updated the xamDataChart styling-points topic to include MarkerSize alongside MarkerStyle
  • Clarified behavior for marker-capable MarkerSeries types:
    • explicit pixel sizing
    • NaN to restore template/default sizing
    • BubbleSeries remains radius-driven

• Release notes

  • Added MarkerSize to the current changelog enhancement entry in:
    • topics/en/changelog.adoc
    • topics/ja/changelog.ja-JP.adoc

• What’s New

  • Added new 2026 Volume 1 release topics in English and Japanese covering:
    • property purpose
    • sizing behavior
    • supported series
    • BubbleSeries exception
  • Linked the new topics from revision history so they appear in the release index

• Localization parity

  • Mirrored all relevant updates in Japanese to keep release and product documentation aligned

<ig:XamDataChart.Series>
    <ig:ScatterLineSeries
        MarkerType="Circle"
        MarkerSize="30" />
</ig:XamDataChart.Series>