docs(technical): dayjs intro & supported plugins

docs/User Guide/User Guide/Scripting/Script API/Day.js.md

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+# Day.js
+Day.js is a date manipulation library that's used by Trilium, but it's also shared with both front-end and back-end scripts. For more information about the library itself, consult the [official documentation](https://day.js.org/en/).
+
+## How to use
+
+The `dayjs` method is provided directly in the `api` global:
+
+```javascript
+const date = api.dayjs();
+api.log(date.format("YYYY-MM-DD"));
+```
+
+## Plugins
+
+Day.js uses a modular, plugin-based architecture. Generally these plugins must be imported, but this process doesn't work inside Trilium scripts due to the use of a bundler.
+
+Since v0.100.0, the same set of plugins is available for both front-end and back-end scripts.
+
+The following Day.js plugins are directly integrated into Trilium:
+
+*   [AdvancedFormat](https://day.js.org/docs/en/plugin/advanced-format)
+*   [Duration](https://day.js.org/docs/en/plugin/duration), since v0.100.0.
+*   [IsBetween](https://day.js.org/docs/en/plugin/is-between)
+*   [IsoWeek](https://day.js.org/docs/en/plugin/iso-week)
+*   [IsSameOrAfter](https://day.js.org/docs/en/plugin/is-same-or-after)
+*   [IsSameOrBefore](https://day.js.org/docs/en/plugin/is-same-or-before)
+*   [QuarterOfYear](https://day.js.org/docs/en/plugin/quarter-of-year)
+*   [UTC](https://day.js.org/docs/en/plugin/utc)
+
+> [!NOTE]
+> If another Day.js plugin might be needed for scripting purposes, feel free to open a feature request for it. Depending on the size of the plugin and the potential use of it inside the Trilium code base, it has a chance of being integrated.