Add basic documentation for labels, including scoped labels (go-gitea#23304)

Part of go-gitea#22974

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Co-authored-by: delvh <dev.lh@web.de>

Author: 2 people

Diff for: docs/content/doc/usage/labels.en-us.md

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+---
+date: "2023-03-04T19:00:00+00:00"
+title: "Usage: Labels"
+slug: "labels"
+weight: 13
+toc: false
+draft: false
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Labels"
+    weight: 13
+    identifier: "labels"
+---
+
+# Labels
+
+You can use labels to classify issues and pull requests and to improve your overview over them.
+
+## Creating Labels
+
+For repositories, labels can be created by going to `Issues` and clicking on `Labels`.
+
+For organizations, you can define organization-wide labels that are shared with all organization repositories, including both already-existing repositories as well as newly created ones. Organization-wide labels can be created in the organization `Settings`.
+
+Labels have a mandatory name, a mandatory color, an optional description, and must either be exclusive or not (see `Scoped labels` below).
+
+When you create a repository, you can ensure certain labels exist by using the `Issue Labels` option. This option lists a number of available label sets that are [configured globally on your instance](../customizing-gitea/#labels). Its contained labels will all be created as well while creating the repository.
+
+## Scoped Labels
+
+A scoped label is a label that contains `/` in its name (not at either end of the name). For example labels `kind/bug` and `kind/enhancement` both have scope `kind`. Such labels will display the scope with slightly darker color.
+
+The scope of a label is determined based on the **last** `/`, so for example the scope of label `scope/subscope/item` is `scope/subscope`.
+
+Scoped labels can be marked as exclusive. This ensures at most a single label with the same scope is assigned to an issue or pull request. For example, if `kind/bug` and `kind/enhancement` are marked exclusive, an issue can only be classified as a bug or an enhancement.
+
+## Filtering by Label
+
+Issue and pull request lists can be filtered by label. Selecting multiple labels shows issues and pull requests that have all selected labels assigned.
+
+By holding alt to click the label, issues and pull requests with the chosen label are excluded from the list.