7017 update highlights page for v1.3 (#7051)

part of #7017


### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Wenqi Li <wenqil@nvidia.com>

Author: wyli

docs/source/whatsnew.rst

@@ -6,6 +6,7 @@ What's New
 .. toctree::
    :maxdepth: 1
 
+   whatsnew_1_3.md
    whatsnew_1_2.md
    whatsnew_1_1.md
    whatsnew_1_0.md

docs/source/whatsnew_1_2.md

@@ -1,4 +1,4 @@
-# What's new in 1.2 🎉🎉
+# What's new in 1.2
 
 - Auto3DSeg enhancements and benchmarks
 - nnUNet integration

docs/source/whatsnew_1_3.md

@@ -0,0 +1,23 @@
+# What's new in 1.3 🎉🎉
+
+- Bundle usability enhancements
+- Integrating MONAI Generative into MONAI core
+
+
+# Bundle usability enhancements
+
+Based on the experience of building MONAI model zoo and the feedback from the community,
+MONAI 1.3 provides major enhancements in MONAI Bundle usability. These include:
+- Pythonic APIs for Bundle trying to strike a balance between code readability and workflow standardization;
+- Streamlined Bundle building processes with step-by-step guides to the concepts;
+- Various utility functions for fetching and fine-tuning models from [MONAI Model Zoo](https://github.com/Project-MONAI/model-zoo);
+- Various fixes for Bundle syntax and documentation, improved test coverage across the Bundle module and Model Zoo.
+
+For more details please visit [the Bundle tutorials](https://github.com/Project-MONAI/tutorials/tree/main/bundle) and
+[the Model Zoo demos](https://github.com/Project-MONAI/tutorials/tree/main/model_zoo).
+
+# Integrating MONAI Generative into MONAI Core
+
+Main modules developed at [MONAI GenerativeModels](https://github.com/Project-MONAI/GenerativeModels)
+are being ported into the core codebase, allowing for consistent maintenance and release of the key components for generative AI.
+As a starting point, loss functions and metrics are integrated into this version.