fix: change some /themes dead links (#1)

Author: julien-deramond

test/inputs/demo.md

@@ -243,8 +243,8 @@ theme: seriph
 
 </div>
 
-Read more about [How to use a theme](https://sli.dev/themes/use.html) and
-check out the [Awesome Themes Gallery](https://sli.dev/themes/gallery.html).
+Read more about [How to use a theme](https://sli.dev/guide/theme-addon#use-theme) and
+check out the [Awesome Themes Gallery](https://sli.dev/resources/theme-gallery).
 
 ---
 preload: false

test/outputs/demo_config1.md

@@ -245,8 +245,8 @@ theme: seriph
 
 </div>
 
-Read more about [How to use a theme](https://sli.dev/themes/use.html) and
-check out the [Awesome Themes Gallery](https://sli.dev/themes/gallery.html).
+Read more about [How to use a theme](https://sli.dev/guide/theme-addon#use-theme) and
+check out the [Awesome Themes Gallery](https://sli.dev/resources/theme-gallery).
 
 ---
 preload: false

test/outputs/demo_default.md

@@ -245,8 +245,8 @@ theme: seriph
 
 </div>
 
-Read more about [How to use a theme](https://sli.dev/themes/use.html) and
-check out the [Awesome Themes Gallery](https://sli.dev/themes/gallery.html).
+Read more about [How to use a theme](https://sli.dev/guide/theme-addon#use-theme) and
+check out the [Awesome Themes Gallery](https://sli.dev/resources/theme-gallery).
 
 ---
 preload: false

test/outputs/demo_noEmbed.md

@@ -243,8 +243,8 @@ theme: seriph
 
 </div>
 
-Read more about [How to use a theme](https://sli.dev/themes/use.html) and
-check out the [Awesome Themes Gallery](https://sli.dev/themes/gallery.html).
+Read more about [How to use a theme](https://sli.dev/guide/theme-addon#use-theme) and
+check out the [Awesome Themes Gallery](https://sli.dev/resources/theme-gallery).
 
 ---
 preload: false