Add language tags to code blocks

Code blocks should specify the language the code is in, so we
can enforce a markdownlint rule.

doc/administration/auth/crowd.md

@@ -78,7 +78,7 @@ On the sign in page there should now be a Crowd tab in the sign in form.
 
 If you see an error message like the one below when you sign in after Crowd authentication is configured, you may want to consult the Crowd administrator for the Crowd log file to know the exact cause:
 
-```
+```plaintext
 could not authorize you from Crowd because invalid credentials
 ```
 

doc/administration/auth/how_to_configure_ldap_gitlab_ce/index.md

@@ -56,7 +56,7 @@ Using PowerShell you can output the **OU** structure as a table (_all names are
 Get-ADObject -LDAPFilter "(objectClass=*)" -SearchBase 'OU=GitLab INT,DC=GitLab,DC=org' -Properties CanonicalName | Format-Table Name,CanonicalName -A
 ```
 
-```
+```plaintext
 OU                CanonicalName
 ----              -------------
 GitLab INT        GitLab.org/GitLab INT
@@ -109,7 +109,7 @@ The two Active Directory specific values are `active_directory: true` and `uid:
 
 ### Example `gitlab.rb` LDAP
 
-```
+```ruby
 gitlab_rails['ldap_enabled'] = true
 gitlab_rails['ldap_servers'] = {
 'main' => {
@@ -186,7 +186,7 @@ ldapsearch -D "CN=GitLabSRV,CN=Users,DC=GitLab,DC=org" -w Password1 -p 389 -h ad
 
 **Full output of `ldapsearch` command:** - Filtering for _CN=Leroy Fox_
 
-```
+```plaintext
 # LDAPv3
 # base <OU=GitLab INT,DC=GitLab,DC=org> with scope subtree
 # filter: CN=Leroy Fox

doc/administration/auth/jwt.md

@@ -45,7 +45,7 @@ JWT will provide you with a secret key for you to use.
 
    For installation from source:
 
-   ```
+   ```yaml
    - { name: 'jwt',
        args: {
          secret: 'YOUR_APP_SECRET',

doc/administration/auth/smartcard.md

@@ -131,14 +131,14 @@ attribute. As a prerequisite, you must use an LDAP server that:
    - The additional NGINX server context must be configured to run on a different
      port:
 
-     ```
+     ```plaintext
      listen *:3444 ssl;
      ```
 
    - The additional NGINX server context must be configured to require the client
      side certificate:
 
-     ```
+     ```plaintext
      ssl_verify_depth 2;
      ssl_client_certificate /etc/ssl/certs/CA.pem;
      ssl_verify_client on;
@@ -147,14 +147,14 @@ attribute. As a prerequisite, you must use an LDAP server that:
    - The additional NGINX server context must be configured to forward the client
      side certificate:
 
-     ```
+     ```plaintext
      proxy_set_header    X-SSL-Client-Certificate    $ssl_client_escaped_cert;
      ```
 
    For example, the following is an example server context in an NGINX
    configuration file (eg. in `/etc/nginx/sites-available/gitlab-ssl`):
 
-   ```
+   ```plaintext
    server {
        listen *:3444 ssl;