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>

Author: SISheogorath

docs/guides/auth.md

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+Authentication guide - GitHub
+===
+
+1. Sign-in or sign-up for a GitHub account
+2. Navigate to developer settings in your GitHub account [here](https://github.com/settings/developers) and select the "OAuth Apps" tab
+3. Click on the **New OAuth App** button, to create a new OAuth App:
+
+![create-oauth-app](../images/auth/create-oauth-app.png)
+
+4. Fill out the new OAuth application registration form, and click **Register Application**
+
+![register-oauth-application-form](../images/auth/register-oauth-application-form.png)
+
+*Note: The callback URL is <your-hackmd-url>/auth/github/callback*
+
+5. After successfully registering the application, you'll receive the Client ID and Client Secret for the application
+
+![application-page](../images/auth/application-page.png)
+
+6. Add the Client ID and Client Secret to your config.json file or pass them as environment variables
+    * config.json:
+      ````javascript
+      {
+        "production": {
+          "github": {
+              "clientID": "3747d30eaccXXXXXXXXX",
+              "clientSecret": "2a8e682948eee0c580XXXXXXXXXXXXXXXXXXXXXX"
+          }
+        }
+      }
+      ````
+    * environment variables:
+      ````
+      HMD_GITHUB_CLIENTID=3747d30eaccXXXXXXXXX
+      HMD_GITHUB_CLIENTSECRET=2a8e682948eee0c580XXXXXXXXXXXXXXXXXXXXXX
+      ````

docs/guides/auth/github.md

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+# 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)

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

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+Authentication guide - SAML (OneLogin)
+===
+
+1. Sign-in or sign-up for an OneLogin account. (available free trial for 2 weeks)
+2. Go to the administration page.
+3. Select the **APPS** menu and click on the **Add Apps**.
+
+![onelogin-add-app](../images/auth/onelogin-add-app.png)
+
+4. Find "SAML Test Connector (SP)" for template of settings and select it.
+
+![onelogin-select-template](../images/auth/onelogin-select-template.png)
+
+5. Edit display name and icons for OneLogin dashboard as you want, and click **SAVE**.
+
+![onelogin-edit-app-name](../images/auth/onelogin-edit-app-name.png)
+
+6. After that other tabs will appear, click the **Configuration**, and fill out the below items, and click **SAVE**.
+    * RelayState: The base URL of your hackmd, which is issuer. (last slash is not needed)
+    * ACS (Consumer) URL Validator: The callback URL of your hackmd. (serverurl + /auth/saml/callback)
+    * ACS (Consumer) URL: same as above.
+    * Login URL: login URL(SAML requester) of your hackmd. (serverurl + /auth/saml)
+
+![onelogin-edit-sp-metadata](../images/auth/onelogin-edit-sp-metadata.png)
+
+7. The registration is completed. Next, click **SSO** and copy or download the items below.
+    * X.509 Certificate: Click **View Details** and **DOWNLOAD** or copy the content of certificate ....(A)
+    * SAML 2.0 Endpoint (HTTP): Copy the URL ....(B)
+
+![onelogin-copy-idp-metadata](../images/auth/onelogin-copy-idp-metadata.png)
+
+8. In your hackmd server, create IdP certificate file from (A)
+9. Add the IdP URL (B) and the Idp certificate file path to your config.json file or pass them as environment variables.
+    * config.json:
+      ````javascript
+      {
+        "production": {
+          "saml": {
+            "idpSsoUrl": "https://*******.onelogin.com/trust/saml2/http-post/sso/******",
+            "idpCert": "/path/to/idp_cert.pem"
+          }
+        }
+      }
+      ````
+    * environment variables
+      ````
+      HMD_SAML_IDPSSOURL=https://*******.onelogin.com/trust/saml2/http-post/sso/******
+      HMD_SAML_IDPCERT=/path/to/idp_cert.pem
+      ````
+10. Try sign-in with SAML from your hackmd sign-in button or OneLogin dashboard (like the screenshot below).
+
+![onelogin-use-dashboard](../images/auth/onelogin-use-dashboard.png)