Guides (API): Some additional API token info

[felixyz]

UPDATE: Ignore everything below. All that remains is a small extra tip on where to find your API token and a curl example.

Description

The API guide states that the API token should be passed in through the Authentication header. I discovered this doesn't work, and from this line it looks like the correct header is X-Spree-Token, which also works in practice for me.

Either I misunderstood something, or the guides are wrong.

I also added a curl example and spelled out where people can find their admin token.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)

[kennyadsl]

@felixyz thanks for the contribution. Which Solidus version are you using? Recently, we deprecated X-Spree-Token, so I think guides are right. If the new header is not working for you, it's probably because you are on an old Solidus version, isn't it?

[felixyz]

@kennyadsl 🤦‍♂️ haha you caught me there, now you can infer just how far behind I am with my updates.

How about the other stuff? It's tiny but perhaps worth merging?

[kennyadsl]

Sure, I like the rest of the PR. This also needs a rebase to fix pending CI checks I guess. Thanks Felix!

[kennyadsl]

@felixyz do you have time to complete this PR or it's better to close it?

[felixyz]

@kennyadsl Should be ready now.

[kennyadsl]

Thanks, Felix!

[aldesantis]

Looking good, thanks!