DOC: provide example of creating an index on a many-many table

[sunnysideup]

Description

It is not so easy to work out how to add an index to a many-many Extra Field. This provides an example

Issues

No issue yet.

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  TBC

[GuySartorelli]

I don't think this is an appropriate way to add an index for this purpose. It's fine as a workaround, but if there's no existing configuration that can be used, a new configuration should probably be added for this purpose.