landing-page: add overview video

* Add overview video
* Add CSS for YouTube video embed
  * Usage:
    * on YouTube page, click Share->Embed and copy code into Docusuarus
    * remove the width and height properties
    * add `class="youtube-embed"`

Signed-off-by: Mike Szczys <mike@golioth.io>

Author: szczys

docs/landing_page.md

@@ -16,6 +16,8 @@ need from the cloud, from establishing secure connections and providing
 over-the-air updates, to managing data to/from your fleet and connecting that to
 any cloud service you may need.
 
+<iframe class="youtube-embed" src="https://www.youtube.com/embed/CgQg6ifrWfU?si=9MKtgs4jlCvQLWfz" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
+
 ## Get started today
 
 Our [Quickstart Guide](/getting-started/) gets you set up with a Golioth account

src/css/custom.css

@@ -146,3 +146,8 @@ html[data-theme="dark"] {
     flex-direction: column;
   }
 }
+
+iframe.youtube-embed {
+  width: 100%;
+  aspect-ratio: 16 / 9;
+}