Merge pull request #28393 from Kumataro:develop/doc_build_config_avif

doc: update image codec configuration reference (add AVIF, fix JPEG XL) #28393

This PR updates the build configuration documentation to:
- Add AVIF to the supported format list (it was missing).
- Clarify that some codecs (AVIF, JPEG XL) do not support BUILD_* options because OpenCV does not bundle their source code.
- Update the description to include "write" capabilities for these formats.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

doc/tutorials/introduction/config_reference/config_reference.markdown

@@ -317,7 +317,7 @@ Following formats can be read by OpenCV without help of any third-party library:
 | [PFM](https://en.wikipedia.org/wiki/Netpbm#File_formats) | `WITH_IMGCODEC_PFM` | _ON_ |
 | [GIF](https://en.wikipedia.org/wiki/GIF) | `WITH_IMGCODEC_GIF` | _ON_ |
 
-### PNG, JPEG, TIFF, WEBP, JPEG 2000, EXR, JPEG XL support
+### PNG, JPEG, TIFF, WEBP, JPEG 2000, EXR, JPEG XL, AVIF support
 
 | Formats | Library | Option | Default | Force build own |
 | --------| ------- | ------ | ------- | --------------- |
@@ -331,12 +331,16 @@ Following formats can be read by OpenCV without help of any third-party library:
 |^| [JasPer](https://en.wikipedia.org/wiki/JasPer) | `WITH_JASPER` | _ON_ (see note) | `BUILD_JASPER` |
 | [OpenEXR](https://en.wikipedia.org/wiki/OpenEXR) || `WITH_OPENEXR` | _ON_ | `BUILD_OPENEXR` |
 | [JPEG XL](https://en.wikipedia.org/wiki/JPEG_XL) || `WITH_JPEGXL` | _ON_ | Not supported. (see note) |
+| [AVIF](https://en.wikipedia.org/wiki/AVIF) || `WITH_AVIF` | _ON_ | Not supported. (see note) |
 
-All libraries required to read images in these formats are included into OpenCV and will be built automatically if not found at the configuration stage. Corresponding `BUILD_*` options will force building and using own libraries, they are enabled by default on some platforms, e.g. Windows.
+Most library source codes required to read/write images in these formats are bundled into OpenCV and will be built automatically if not found at the configuration stage
+(except for some codecs that require external libraries, e.g. JPEG XL and AVIF).
+Corresponding BUILD_* options will force building and using the bundled libraries; they are enabled by default on some platforms, e.g. Windows.
 
 @note (All) Only one library for each image format can be enabled(e.g. In order to use JasPer for JPEG 2000 format, OpenJPEG must be disabled).
 @note (JPEG 2000) OpenJPEG have higher priority than JasPer which is deprecated.
-@note (JPEG XL) OpenCV doesn't contain libjxl source code, so `BUILD_JPEGXL` is not supported.
+@note (JPEG XL) OpenCV doesn't contain libjxl source code, so `BUILD_JPEGXL` is not supported. Users must provide a system-wide installation of libjxl.
+@note (AVIF) OpenCV doesn't contain libavif source code, so `BUILD_AVIF` is not supported. Users must provide a system-wide installation of libavif.
 
 @warning OpenEXR ver 2.2 or earlier cannot be used in combination with C++17 or later. In this case, updating OpenEXR ver 2.3.0 or later is required.