style: linting markdown files

Linting markdown files according to default remark-lint configuration.

Files inside the `public` directory were not linted.

Signed-off-by: oupala <oupala@users.noreply.github.com>

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

@@ -1,7 +1,6 @@
-GitLab (self-hosted)
-===
+# GitLab (self-hosted)
 
-**Note:** *This guide was written before the renaming. Just replace `HackMD` with `CodiMD` in your mind :smile: thanks!*
+*Note:* This guide was written before the renaming. Just replace `HackMD` with `CodiMD` in your mind :smile: thanks!
 
 1. Sign in to your GitLab
 2. Navigate to the application management page at `https://your.gitlab.domain/admin/applications` (admin permissions required)
@@ -14,10 +13,9 @@ GitLab (self-hosted)
 
 ![Application: HackMD](../../images/auth/gitlab-application-details.png)
 
-
 6. In the `docker-compose.yml` add the following environment variables to `app:` `environment:`
 
-```
+```Dockerfile
 - CMD_DOMAIN=your.codimd.domain
 - CMD_URL_ADDPORT=true
 - CMD_PROTOCOL_USESSL=true