docs(ai): add skills documentation

based on the BEP-0013 add the necessary docs for using and
contributing AI skills

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

docs/ai/skills.md

@@ -0,0 +1,93 @@
+---
+id: skills
+title: AI Skills
+description: Reusable AI skills for common Backstage development tasks.
+---
+
+Backstage publishes a set of curated _AI skills_ — self-contained guidance files that teach an AI coding assistant how to perform common Backstage engineering tasks. Skills are published to a [well-known endpoint](https://backstage.io/.well-known/skills/) on `backstage.io` and can be installed into your repository with the [`skills.sh`](https://skills.sh/) tool.
+
+## Installing Skills
+
+You need [Node.js](https://nodejs.org/) to run `npx`.
+
+```bash
+npx skills add https://backstage.io
+```
+
+This command reads the published index from `https://backstage.io/.well-known/skills/index.json` and allows you to select which of the available skills you want to install into your repository.
+
+### Where skills are installed
+
+`skills.sh` copies skill files into your repository under a directory it manages (typically `.github/skills/` or a similar location depending on your configuration). Refer to the [`skills.sh` documentation](https://skills.sh/) for details on target paths and how to customize them.
+
+After installation, you can modify the installed files to adapt them to your project's conventions. Subsequent updates from `npx skills add` will offer to merge upstream changes.
+
+### Using Skills with Your AI Assistant
+
+Once a skill is installed in your repository, attach or reference the relevant `SKILL.md` file when starting a task with your AI coding assistant. Most AI assistants in editors such as VS Code will automatically pick up instruction files that are committed to your repository.
+
+For example, when migrating MUI imports in a plugin, include the `mui-to-bui-migration` skill so the assistant follows the correct component mapping and import patterns.
+
+## Contributing New Skills
+
+Skills are authored in the Backstage monorepo at `docs/.well-known/skills/`. Each skill lives in its own subdirectory and must include a `SKILL.md` file as the primary entry point.
+
+### Skill directory layout
+
+```text
+docs/.well-known/skills/
+  index.json                        # Published index of all skills
+  <skill-name>/
+    SKILL.md                        # Primary skill entry point (required)
+    <optional-supporting-files>
+```
+
+### Writing a SKILL.md
+
+A `SKILL.md` file must include a YAML front matter block with the following fields:
+
+```markdown
+---
+name: <skill-name>
+description: <short description used in the published index>
+---
+
+# Skill Title
+
+Introductory paragraph explaining when and why to use this skill.
+
+...
+```
+
+The `name` must match the directory name. The `description` is shown to users when they browse or install skills and should be one to two sentences describing the task the skill covers.
+
+Keep skills focused on a single, well-defined task. A skill that tries to cover too many scenarios is harder to use effectively. Prefer concrete, step-by-step guidance, working code examples, and explicit notes about common pitfalls.
+
+### Registering the skill in the index
+
+Add an entry to `docs/.well-known/skills/index.json`:
+
+```json
+{
+  "skills": [
+    {
+      "name": "<skill-name>",
+      "description": "<same description as in SKILL.md front matter>",
+      "files": ["SKILL.md"]
+    }
+  ]
+}
+```
+
+If your skill includes additional supporting files, list each one in the `files` array.
+
+### Review process
+
+All changes to skills go through the standard Backstage pull request process. When authoring or reviewing a skill, consider:
+
+- **Accuracy** — Does the skill reflect current Backstage APIs and conventions?
+- **Completeness** — Does it cover the most common cases a developer will encounter?
+- **Safety** — Does it avoid patterns that could introduce security or correctness issues?
+- **Scope** — Is the skill focused on a single task, or should it be split?
+
+Skills are part of the published Backstage documentation surface, so they follow the same contribution guidelines as the rest of the docs. See [CONTRIBUTING.md](https://github.com/backstage/backstage/blob/master/CONTRIBUTING.md) for the full contribution process.