docs: Add content-type to allowed headers

[DenisPalnitsky]

It's a documentation update. Cors fails in case when Kratos public is not on the same subdomain as client.
I could be fixed by adding Content-Type to allow headers.
Refer to #1993

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Thank you, this looks great! Could you please sign the CLA (just two clicks), then we can merge it right away!

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Thank you very much for your contribution to the documentation! Due to the way Docusaurus versioning works, you need to make the changes in two locations:

• docs/versioned_docs/version-v{version}/path/to/file
• docs/docs/path/to/file

Without making them in both locations, the changes will most likely be lost when releasing another version. Thank you! :)

[DenisPalnitsky]

Thank you, this looks great! Could you please sign the CLA (just two clicks), then we can merge it right away!

Done.
Also added the same fix to unversioned docs.