Use createNodeId for gatsby-source-contentful

[KyleAMathews]

People think they can query contentful nodes with the Contentful IDs but get tripped up because sometimes they can't due to needing to left-pad ids starting with numbers. Let's just make all IDs UUIDs to prevent that confusion #4001

[mxlje]

Maybe i‘m missing something, how would we see those IDs? And how would they be persistent?

Right now I have to run IDs through the fixId function before I can actually use them, but at least it explicitly references the entry by its canonical (contentful) ID in the code.

[pieh]

@mxlje use contentful_id field instead of id field

[mxlje]

Ah, of course! Thank you.

[anjawaldbauer]

I might be mistake here, but I think the use of contentful_id is limited to the case where you only have one locale. When you use the contentful_id on a localized node, i.e. where the id ends in __de (for the de localization) you will retrieve the default - e.g. English - node instead of the German (de) node.

[pieh]

This was already done in #3807 and later updated in #5167