Add LVGL chart widget documentation

[pastukhov]

Summary

• add a new chart widget section to LVGL widgets docs
• document main chart options (type, update_mode, point_count, axis ranges, series)
• document chart actions: lvgl.chart.update and lvgl.chart.series.update
• add a full YAML example showing periodic updates

Related

• [lvgl] Add chart widget and series update action esphome#13987

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[Copilot]

Pull request overview

This PR adds comprehensive documentation for the LVGL chart widget, which enables users to visualize data series as line, bar, or scatter charts on LVGL-based displays. The documentation covers configuration options, actions for updating charts and series, and includes a complete working example showing periodic chart updates.

Changes:

• Added chart widget documentation with configuration variables for chart type, update mode, point count, division lines, axis ranges, and series definitions
• Documented two chart-specific actions: lvgl.chart.update for updating chart properties and lvgl.chart.series.update for updating series data
• Included a full YAML example demonstrating chart initialization and periodic value updates using intervals