Update docs for v0.28

[AlexeyMz]

• Write a proper "concept" pages for graph model, data provider, event system, command history and workspace context;
• Add "concept" pages for the design system and i18n;
• Provide info on styling in the "component" doc pages;
• Add example for getting a CanvasApi instance;
• Add example for opening the `ConnectionsMenu``;
• Add example for the `InstancesSearch``;
• Add example for the `Navigator``;
• Add example for custom search section for UnifiedSearch;
• Separate Rdf playground into RDF Explorer and Graph Authoring;
• Fix styles for layout panels;
• Change docs status sign from alpha to beta;
• Update ExampleMetadata for the next version;
• Use "reactodia.github.io" to fetch example graph (org ontology);
• Use source map loader to include resolve Reactodia source in the debugger;
• Update dependency to @reactodia/workspace@0.28.0.