Add add-repo doc (#1443)

* Add add-repo doc

* Edits

Author: theletterf

docs/_docset.yml

@@ -31,6 +31,7 @@ toc:
       - file: on-the-web.md
       - file: move.md
       - file: redirects.md
+      - file: add-repo.md
   - folder: migration
     children:
       - file: index.md

docs/contribute/add-repo.md

@@ -0,0 +1,83 @@
+# Add a new repository to the docs
+
+Elastic documentation is built from many assembled repositories using `docs-assembler`. Adding a new repository requires making the assembly process aware of its existence.
+
+Follow these instructions to add a new docs repository.
+
+## Prerequisites
+
+The new docs repository needs to satisfy these requirements:
+
+- The repository must have a `docs` folder in the root.
+- The `docs` folder must contain a valid [`docset.yml` file](../configure/content-set/navigation.md) and Markdown files.
+- Markdown files within the `docs` folder that follow the V3 format. Refer to [Syntax](../syntax/index.md).
+- The repository must be within the Elastic organization on `GitHub` and public.
+
+## Add the repository
+
+Follow these instructions to add a new repository to the docs.
+
+:::::{stepper}   
+
+::::{step} Add the repo to docs-infra
+
+Add the repo to the list of repositories that can upload to the Link index by editing the [repositories.yml](https://github.com/elastic/docs-infra/blob/main/modules/aws-github-actions-oidc-roles/repositories.yml) file.
+
+For example, to add the fictitious `elastic/yadda-docs` repository:
+
+```yaml
+repositories:
+    - name: elastic/yadda-docs # Added for testing purposes
+```
+
+::::
+
+::::{step} Add the workflow actions to the repository
+
+Add the following actions to the `.github/workflows` directory of your repo:
+
+- https://github.com/elastic/docs-builder/blob/main/.github%2Fworkflows%2Fpreview-build.yml
+- https://github.com/elastic/docs-builder/blob/main/.github/workflows/preview-cleanup.yml
+
+Then, successfully run a docs build on the `main` branch. This is a requirement. For example, you can merge a docs pull request to `main` after adding the workflow actions.
+
+::::
+
+::::{step} Add the repository to the assembler and navigation configs
+
+Edit the [assembler.yml](https://github.com/elastic/docs-builder/blob/main/src/tooling/docs-assembler/assembler.yml) file to add the repository. Refer to [assembler.yml](../configure/site/content.md) for more information.
+
+For example, to add the `elastic/yadda-docs` repository:
+
+```yaml
+references:
+  yadda-docs:
+```
+
+Then, edit the [navigation.yml](https://github.com/elastic/docs-builder/blob/main/src/tooling/docs-assembler/navigation.yml) file to add the repository to the navigation.
+
+For example, to add the `elastic/yadda-docs` repository under **Reference**:
+
+```yaml
+  #############
+  # REFERENCE #
+  #############
+  - toc: reference
+    path_prefix: reference
+    children:
+      # Yadda
+      # ✅ https://github.com/elastic/yadda-docs/blob/main/docs/toc.yml
+      - toc: yadda-docs://
+        path_prefix: reference/yadda
+```
+
+::::
+:::::
+
+## Add .artifacts to .gitignore
+
+For a more comfortable local `docs-builder` experience, add the following line to the `.gitignore` file of the repo:
+
+```
+docs/.artifacts 
+```