State that a Service API key is required for Spectre imports

[PolygonError]

States the required API key type required to import data from the Spectre API, as referenced in issue firefly-iii/firefly-iii#8381

Changes in this pull request:

• Modified docs/docs/how-to/data-importer/import/salt-edge.md to explain how to create an application API key on Salt Edge and added a warning that a Service API key must be made.
• Also made hyperlinking consistent on line 17 with the rest of the document

@JC5

[JC5]

I like the idea, but I'm not a fan of this version, to be honest. Two things stand out to me:

1. You call it a "application API key" in the text, and in the note block insist that users must not create "an App API key".
2. I don't think a note block is necessary at all. Just say "service API key" when you mention the app key.

[JC5]

Closing due to lack of response.