Split authentication guides into multiple documents

Splitting the documentation should provide an easier access to the
documentation people searching for and result in less merge conflicts
when adding new documentation here.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

docs/guides/auth/gitlab-self-hosted.md

@@ -0,0 +1,30 @@
+# GitLab (self-hosted)
+===
+
+1. Sign in to your GitLab
+2. Navigate to the application management page at `https://your.gitlab.domain/admin/applications` (admin permissions required)
+3. Click **New application** to create a new application and fill out the registration form:
+
+![New GitLab application](images/auth/gitlab-new-application.png)
+
+4. Click **Submit**
+5. In the list of applications select **HackMD**. Leave that site open to copy the application ID and secret in the next step.
+
+![Application: HackMD](images/auth/gitlab-application-details.png)
+
+
+6. In the `docker-compose.yml` add the following environment variables to `app:` `environment:`
+
+```
+- HMD_DOMAIN=your.hackmd.domain
+- HMD_URL_ADDPORT=443
+- HMD_PROTOCOL_USESSL=true
+- HMD_GITLAB_BASEURL=https://your.gitlab.domain
+- HMD_GITLAB_CLIENTID=23462a34example99XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+- HMD_GITLAB_CLIENTSECRET=5532e9dexamplXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+```
+
+7. Run `docker-compose up -d` to apply your settings.
+8. Sign in to your HackMD using your GitLab ID:
+
+![Sign in via GitLab](images/auth/gitlab-sign-in.png)