docs: GitLab tutorial and CI/CD component reference (#72)

docs/tutorials/github.md

@@ -1,6 +1,6 @@
-# GitHub
+# Getting started on GitHub
 
-In this tutorial we show how to install `releaser-pleaser` in your GitHub project.
+In this tutorial you will learn how to set up `releaser-pleaser` in your GitHub project with GitHub Actions.
 
 ## 1. Repository Settings
 
@@ -52,7 +52,7 @@ jobs:
       pull-requests: write
     steps:
       - name: releaser-pleaser
-        uses: apricote/releaser-pleaser@v0.2.0
+        uses: apricote/releaser-pleaser@v0.4.0
 ```
 
 ## 3. Release Pull Request

docs/tutorials/gitlab-access-token.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:31b485bbe031443c4bfa0d39514dc7e5d524925aa877848def93ee40f69a1897
+size 146496

docs/tutorials/gitlab-settings-merge-method.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:b853625854582a66ab2438f11e6001a88bcb276225abed536ba68617bde324db
+size 57583

docs/tutorials/gitlab-settings-squash.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:0ce9b9826229851e961ef55d91cb9ba91ca9ca4d955a932d9ff6b10d04788c29
+size 41048

docs/tutorials/gitlab.md

@@ -0,0 +1,88 @@
+# Getting started on GitLab
+
+In this tutorial you will learn how to set up `releaser-pleaser` in your GitLab project with GitLab CI.
+
+> In `releaser-pleaser` documentation we mostly use "Pull Request" (GitHub wording) instead of "Merge Request" (GitLab wording). The GitLab-specific pages are an exception and use "Merge Request".
+
+## 1. Project Settings
+
+### 1.1. Merge Requests
+
+`releaser-pleaser` requires _Fast-forward merges_ and _squashing_. With other merge options it can not reliably find the right merge request for every commit on `main`.
+
+Open your project settings to page _Merge Requests_:
+
+> `https://gitlab.com/YOUR-PATH/YOUR-PROJECT/-/settings/merge_requests`
+
+In the "Merge method" section select "Fast-forward merge":
+
+![Screenshot of the required merge method settings](./gitlab-settings-merge-method.png)
+
+In the "Squash commits when merging" section select "Require":
+
+![Screenshot of the required squash settings](./gitlab-settings-squash.png)
+
+## 2. API Access Token
+
+`releaser-pleaser` uses the GitLab API to create the [release merge request](../explanation/release-pr.md) and subsequent releases for you. The default `GITLAB_TOKEN` available in CI jobs does not have enough permissions for this, so we need to create an Access Token and make it available in a CI variable.
+
+## 2.1. Create Project Access Token
+
+Open your project settings to page _Access tokens_:
+
+> `https://gitlab.com/YOUR-PATH/YOUR-PROJECT/-/settings/access_tokens`
+
+Create a token with these settings:
+
+- **Name**: `releaser-pleaser`
+- **Role**: `Maintainer`
+- **Scopes**: `api`, `read_repository`, `write_repository`
+
+Copy the created token for the next step.
+
+![Screenshot of the access token settings](./gitlab-access-token.png)
+
+## 2.2. Save token in CI variable
+
+Open your project settings to page _CI/CD_:
+
+> `https://gitlab.com/YOUR-PATH/YOUR-PROJECT/-/settings/ci_cd`
+
+In the section "Variables" click on the "Add variable" button to open the form for a new variable. Use these settings to create the new variable:
+
+- **Type**: Variable
+- **Visibility**: Masked
+- **Flags**: Uncheck "Protect variable" if your `main` branch is not protected
+- **Key**: `RELEASER_PLEASER_TOKEN`
+- **Value**: The project access token from the previous step
+
+## 3. GitLab CI/CD
+
+`releaser-pleaser` is published as a [GitLab CI/CD Component](https://docs.gitlab.com/ee/ci/components/): https://gitlab.com/explore/catalog/apricote/releaser-pleaser
+
+Create or open your `.gitlab-ci.yml` and add the following include to your configuration:
+
+```yaml
+stages: [build]
+
+include:
+  - component: $CI_SERVER_FQDN/apricote/releaser-pleaser/run@v0.4.0-beta.1
+    inputs:
+      token: $RELEASER_PLEASER_TOKEN
+```
+
+> You can set the `stage` input if you want to run `releaser-pleaser` during a different stage.
+
+## 4. Release Merge Request
+
+Once the `releaser-pleaser` job runs for the first time, you can check the logs to see what it did.
+If you have releasable commits since the last tag, `releaser-pleaser` opens a release merge request for the proposed release.
+
+Once you merge this merge request, `releaser-pleaser` automatically creates a Git tag and GitLab Release with the proposed version and changelog.
+
+## Related Documentation
+
+- **Explanation**
+  - [Release Pull Request](../explanation/release-pr.md)
+- **Reference**
+  - [GitLab CI/CD Component](../reference/gitlab-cicd-component.md)