docs: document Axios header configuration for AJAX

[asad4230]

Added instructions on how to set the X-Requested-With header globally when using Axios in the AJAX documentation.

[mergeable]

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[neznaika0]

Delete one newline.

Can you add a more complete example? Check out the fetch api

[michalsn]

I would like to merge this as-is for now and then refactor the entire page afterward. Adding an Axios section makes sense, but Axios is already mentioned in a few places, so this seems like a good start.