Add warning for AVIF image format

[djibe]

Nowhere in the doc the absence of AVIF processing is stated.

[jmooring]

In general, documentation should state what something does, not what it doesn't do.

Add a bulleted list of raster image formats supported by Hugo's imaging pipeline. Place it beneath this:

[djibe]

As AVIF is now a common web file format, I thought it was supported.

I lost 40 minutes struggling and Hugo's search engine did not help. I think stating this case is important.

I read in https://gohugo.io/functions/images/config/ that "Supported image formats include GIF, JPEG, PNG, TIFF, and WebP."
Should I add there a message to add an AVIF keyword ?

[bep]

I lost 40 minutes struggling and Hugo's search engine did not help. I think stating this case is important.

It took me 1/2 second to NOT find it:

As AVIF is now a common web file format, I thought it was supported.

We support the formats Go(lang) have decoders for. AVIF is not one of them (common or not).