Feature: association between outings

**Community discussion about this feature (mandatory)**
[Link to where this feature has been validated](https://forum.camptocamp.org/t/associer-1-ou-x-sorties-entre-elles/296939/15)

**Is your feature request related to a problem? Please describe.**
Easiest way to get access to multiple outings linked to a ski raid for example.

**Describe the solution you'd like**
have the possibility to associate documents "outings" one woith each other to get the complete list of associated outings on the left bar of the outing document

**Describe alternatives you've considered**
see forum link

**Additional context**
see forum link

Start with the linked forum discussion about associating outings and inspect the existing outing document UI. Define how outings are associated and displayed in the left bar; done means an outing can expose the complete list of associated outings.

Written by the indexing model from the issue text.