fix(docs): fix dead links and code examples (#905)

Author: yggg

docs/articles/auth-guard.md

@@ -105,7 +105,7 @@ Let's modify our guard a bit to reflect this logic:
 import { Injectable } from '@angular/core';
 import { CanActivate, Router } from '@angular/router';
 import { NbAuthService } from '@nebular/auth';
-import { tap } from 'rxjs/operators/tap';
+import { tap } from 'rxjs/operators';
 
 @Injectable()
 export class AuthGuard implements CanActivate {

docs/articles/auth-install.md

@@ -109,7 +109,7 @@ export const routes: Routes = [
 
 ## Enable Styles
 
-Last but not least - install the component styles into your styles.scss ([more details](docs/guides/enabling-theme-system)):
+Last but not least - install the component styles into your styles.scss ([more details](docs/guides/enable-theme-system)):
 
 ```scss
 @import '~@nebular/auth/styles/all'; // or @import '~@nebular/auth/styles/{theme-name}';

docs/articles/concept-theme-system.md

@@ -76,7 +76,7 @@ List of component style variables is specified in the component documentation, f
 
 ## Variables Usage
 
-Now, if you want to use the variables in your custom style files, all you need to do (of course, after the [successful setup of the Theme System](docs/guides/enabling-theme-system) is to call `nb-theme(var-name)` function:
+Now, if you want to use the variables in your custom style files, all you need to do (of course, after the [successful setup of the Theme System](docs/guides/enable-theme-system) is to call `nb-theme(var-name)` function:
 
 ```scss
 @import '../../../@theme/styles/themes';

docs/articles/enabling-theme-system.md

@@ -13,11 +13,10 @@
 
 1) Then you just need to include a CSS file of a theme you want to use into your `.angular-cli.json` file like this:
 
-```scss
+```json5
 "styles": [
-        "../node_modules/@nebular/theme/styles/prebuilt/cosmic.css", // or default.css
-      ],
-
+  "../node_modules/@nebular/theme/styles/prebuilt/cosmic.css", // or default.css
+],
 ```
 
 And that is all. In the future, if you need any of the advanced features - you can easily start using them by going through the Normal/Advanced setup steps.

docs/articles/security-install.md

@@ -17,7 +17,7 @@ npm i @nebular/security
 ## Import the module:
 
 ```ts
-import { NbSecurityModule } from '@nebular/security
+import { NbSecurityModule } from '@nebular/security';
 ```
 <hr>
 ## Register it