Add release notes for Luigi 1.19 (SAP#2404)

blog/2021-11-29-release-notes.md

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+---
+title: Luigi v1.19.0
+seoMetaDescription: Release notes for Luigi v1.18.0
+author:
+  - Aleksandra Simeonova
+layout: blog
+---
+
+You can read about the new features in Luigi v1.19.0 in the release notes below.
+
+<!-- Excerpt -->
+
+#### Replacing CSS static value with SAP Fiori 3 variable
+
+We replaced the CSS static value in LeftNav with a SAP Fiori 3 variable. Now, in [Luigi background themes](https://docs.luigi-project.io/docs/luigi-core-api/?section=theming), Fiori 3 variables are used instead of plain colors. To switch theme, you can replace the CSS variables stylesheet.
+
+You can find more information [here](https://github.com/SAP/luigi/pull/2369).
+
+
+#### Node category merging improvements
+
+We improved the Luigi [category property](https://docs.luigi-project.io/docs/navigation-configuration/?section=category). Now, the  first node with a "non-string" category can define the category's configuration and the category config object includes a new property `id`. If defined, it serves as the "collection key" for categorized nodes instead of the `label` property. You can find more information [here](https://github.com/SAP/luigi/pull/2352).
+
+
+#### New Core API function clearNavigationCache
+
+We introduced a new Core API function called `clearNavigationCache`. It allows you to clean nodeData cache for children resolution and titleResolver cache. For more information, see the [documentation](https://docs.luigi-project.io/docs/luigi-core-api/?section=clearnavigationcache) and [PR](https://github.com/SAP/luigi/pull/2383).
+
+#### Bugfixes
+
+For a full list of bugfixes in this release, see our [changelog](https://github.com/SAP/luigi/blob/master/CHANGELOG.md).
+