docs: add proposal for grafana dashboard parent folder management

[aboulay-numspot]

Aim of the merge request:

This MR aims to create a document to propose an evolution on the folder management related to the parent folder.

The suggested new features are:

• Use a folderRef field to target a folder deployed thanks to the GrafanaFolder CR.
• Use a folderUID field to target an existing folder in Grafana.
• Enable or disable folder creation from GrafanaDashboard by adding a field createFolder (default: true for retrocompatibility). This field cannot be false if folderUID and folderRef are not set in the GrafanaDashboard manifest.

Breaking change

There is no breaking change because it is just a proposal document. Nothing is implemented yet.

This should be discussed during a maintainer meeting.

[CLAassistant]

All committers have signed the CLA.

[theSuess]

Outcome from discussing this in our weekly:

we want to only add folderUid and folderRef fields. The createFolder fields is implicitly defined through the presence of the new fields. This means:

• If folder is set and none of the other fields is set (backwards compat. case): Create the folder
• If folderUID or folderRef is set, don't create the folder
• If folderUID or folderRef AND folder is set, the new fields take priority -> don't create the folder

Will clean up the proposal and we can further discuss this async

[aboulay-numspot]

Outcome from discussing this in our weekly:

we want to only add folderUid and folderRef fields. The createFolder fields is implicitly defined through the presence of the new fields. This means:

• If folder is set and none of the other fields is set (backwards compat. case): Create the folder
• If folderUID or folderRef is set, don't create the folder
• If folderUID or folderRef AND folder is set, the new fields take priority -> don't create the folder

Will clean up the proposal and we can further discuss this async

@theSuess Normally, this should be fine now. I let you review a last time the document. :)