[charts] Do not document the usage of DEFAULT_X_AXIS_KEY and DEFAULT_Y_AXIS_KEY

[alexfauquette]

Fix #12531

The root cause of the issue was that:

• if yAxis is not defined, we internally create a default axis configuration with id DEFAULT_Y_AXIS_KEY.
• if yAxis is defined without an id, the auto-generated ids are deaultized-y-axis-0, deaultized-y-axis-1, deaultized-y-axis-2, ...

To fix this, I propose to allow providing props such as rightAxis with empty config which will render the first axis. Which match how series work:

mui-x/packages/x-charts/src/BarChart/BarPlot.tsx

Lines 89 to 100 in 8efee23

	const { xAxis, yAxis, xAxisIds, yAxisIds } = axisData;
	const defaultXAxisId = xAxisIds[0];
	const defaultYAxisId = yAxisIds[0];
	const data = stackingGroups.flatMap(({ ids: groupIds }, groupIndex) => {
	return groupIds.flatMap((seriesId) => {
	const xAxisKey = series[seriesId].xAxisKey ?? defaultXAxisId;
	const yAxisKey = series[seriesId].yAxisKey ?? defaultYAxisId;
	const xAxisConfig = xAxis[xAxisKey];
	const yAxisConfig = yAxis[yAxisKey];

This allow me to remove all references to DEFAULT_X_AXIS_KEY and DEFAULT_Y_AXIS_KEY in the docs

[mui-bot]

Deploy preview: https://deploy-preview-12780--material-ui-x.netlify.app/

Updated pages:

• docs/data/charts/axis/axis.md

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