doc: add page on light clients (#24647)

* Add light client page

* Rm gemfile.lock

* add section on ulc

* update compose file syntax

docker-compose.yml

@@ -1,7 +1,8 @@
-jekyll:
-    image: jekyll/jekyll:latest
-    command: jekyll serve --watch --incremental
-    ports:
-        - 4000:4000
-    volumes:
-        - .:/srv/jekyll
+services:
+  jekyll:
+      image: jekyll/jekyll:latest
+      command: jekyll serve --watch --incremental
+      ports:
+          - 4000:4000
+      volumes:
+          - .:/srv/jekyll