Improve docs for custom getFeatureInfo workflows for WMS and WMTS imagery

[Kanchanbasnet]

Fixes #13380
Adds documentation for the getFeatureInfoFormats option on WebMapServiceImageryProvider and WebMapTileServiceImageryProvider, since custom format/callback usage wasn't previously documented inline.
Changes:

• Updated the getFeatureInfoFormats description in both classes to explain that since WMS/WMTS services return feature info differently, you may need to write your own format and parsing callback
• Added a link to the existing Sandcastle example that shows how to write a custom callback.
  No code was changed, only documentation comments.

[github-actions]

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[ggetz]

I can confirm we have a CLA on file for @Kanchanbasnet!

[ggetz]

Thanks for the PR @Kanchanbasnet! Could you please add yourself to the list in CONTRIBUTORS.md?