Remove Apros-specifics from the code examples in Simantics/{Chart,Subscription}

Remove Apros-specific documentation from Simantics/{Chart,Subscription}.

Also introduce Simantics/Chart functions:

• createNamedChart
• createNamedChartGroup
• chartByPath
• chartGroupByPath
• chartItemsOf
• chartItemByLabel
• chartItemByIndex
• adjustChartItem