update replay_basics

[monkey0head]

• introduce indexer and clearly explain that user and item ids should be converted to service ids with indexer before replay's models training.
• convert ids back to show how to get recommendations with original ids

optional or to become a part of additional notebook:

• describe the differences between DataPraparator and Indexer
• show, how to add new ids (e.g. we converted a part of log and want to add new ids, or we converted the log only and now want to convert features, where not all the items from features are present in log)