Add a shared GH action for docs preview

Author: nkammah

.github/actions/docs-preview/README.md

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+## About
+
+GitHub Action to create a GitHub comment with the docs-preview
+
+* [Usage](#usage)
+  * [Configuration](#configuration)
+* [Customizing](#customizing)
+  * [inputs](#inputs)
+
+## Usage
+
+### Configuration
+
+Given the CI GitHub action:
+
+```yaml
+---
+on:
+  pull_request_target:
+    types: [opened]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  doc-preview:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: elastic/docs/.github/actions/docs-preview@current
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          repo: ${{ github.event.repository.name }}
+          guide-crumb: 'guide/en/observability/master/index.html'
+          pr: ${{ github.event.pull_request.number }}
+          
+```
+
+## Customizing
+
+### inputs
+
+Following inputs can be used as `step.with` keys
+
+| Name              | Type    | Default                     | Description                             |
+|-------------------|---------|-----------------------------|-----------------------------------------|
+| `guide-crumb`     | String  |                             | The URL crumb to append to base doc url |
+| `repo`            | String  |                             | The GitHub repository name without org  |
+| `github-token`    | String  |                             | The GitHub token                        |
+| `pr`              | String  |                             | The GitHub PR                           | 

.github/actions/docs-preview/action.yml

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+name: 'docs-preview'
+description: 'Add docs-preview comment in the given repository.'
+inputs:
+  repo:
+    description: 'the name of the repository'
+    required: true
+  pr:
+    description: 'the pull-request'
+    required: true
+  github-token:
+    description: 'The GitHub access token.'
+    required: true
+  guide-crumb:
+    description: 'the name of the project'
+    required: false
+    default: ''
+runs:
+  using: "composite"
+  steps:
+    - uses: actions/github-script@v7
+      env:
+        REPO: ${{ inputs.repo }}
+        PR: ${{ inputs.pr }}
+        GUIDE_CRUMB: ${{ inputs.guide-crumb }}
+      with:
+        github-token: ${{ inputs.github-token }}
+        script: |
+          const { REPO, PR, GUIDE_CRUMB } = process.env
+
+          const comment = `A documentation preview will be available soon.
+          Help us out by validating the Buildkite preview and reporting issues [here](https://github.com/elastic/docs/issues/new?labels=buildkite-migration,bug).
+
+            - 📚 HTML diff: [Buildkite](https://${REPO}_bk_${PR}.docs-preview.app.elstc.co/diff)  - [Jenkins](https://${REPO}_${PR}.docs-preview.app.elstc.co/diff)
+            - 📙 Guide: [Buildkite](https://${REPO}_bk_${PR}.docs-preview.app.elstc.co/${GUIDE_CRUMB})  -  [Jenkins](https://${REPO}_${PR}.docs-preview.app.elstc.co/${GUIDE_CRUMB})
+            - 🧪 [Buildkite vs Jenkins diff](https://github.com/elastic/built-docs/compare/${REPO}_bk_${PR}..${REPO}_${PR})`;
+
+          github.rest.issues.createComment({
+            issue_number: PR,
+            owner: 'elastic',
+            repo: REPO,
+            body: comment,
+          });

.github/workflows/test-doc-preview.yml

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+---
+name: test-docs-preview
+
+on:
+  workflow_dispatch:
+  pull_request_target:
+    types: [opened]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  doc-preview-pr:
+    if: github.event_name == 'pull_request_target'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: elastic/docs/.github/actions/docs-preview@main
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          repo: ${{ github.event.repository.name }}
+          guide-crumb: 'guide/en/observability/master/index.html'
+          pr: ${{ github.event.pull_request.number }}
+
+  doc-preview-manual:
+    if: github.event_name == 'workflow_dispatch'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: elastic/docs/.github/actions/docs-preview@main
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          repo: ${{ github.event.repository.name }}
+          guide-crumb: 'guide/en/observability/master/index.html'
+          pr: 2842