Backend CLI: --mappings-only and --create-only confusing

The [`index` command](https://github.com/CentreForDigitalHumanities/I-analyzer/blob/develop/documentation/Indexing-corpora.md) includes these two flags (among others):

- `--mappings-only`: the index job should create an index but not populate it with documents
- `--create-only`: the index job should be created but not run

I didn't think of this when I worked on #1759, but `--create-only` sounds like it means "only create the index", i.e. what `--mappings-only` does.

This is not ideal; perhaps these flag should be renamed.

Start with the index command and documentation/Indexing-corpora.md, where --mappings-only and --create-only are described. Compare the flags' actual behavior with their names, decide on unambiguous replacements, and update the command documentation so each option's meaning is clear.

Written by the indexing model from the issue text.