Remove redundant 404 pages, keep only root 404

IONOS serves the root 404.html for all paths. Updated link text
from 'Back to hub' to 'Back to home'.

code/404.html

@@ -1,13 +0,0 @@
----
-title: "Page Not Found"
-section: "code"
-tags: "code"
-description: "The page you are looking for does not exist."
-layout: base.njk
-permalink: /code/404.html
----
-<div class="error-page">
-  <h1>404</h1>
-  <p>The page you are looking for does not exist or has been moved.</p>
-  <a class="back-link" href="/">← Back to code</a>
-</div>

hub/404.html

@@ -9,5 +9,5 @@ permalink: /404.html
 <div class="error-page">
   <h1>404</h1>
   <p>The page you are looking for does not exist or has been moved.</p>
-  <a class="back-link" href="/">← Back to hub</a>
+  <a class="back-link" href="/">← Back to home</a>
 </div>

infra/404.html

@@ -1,13 +0,0 @@
----
-title: "Page Not Found"
-section: "infra"
-tags: "infra"
-description: "The page you are looking for does not exist."
-layout: base.njk
-permalink: /infra/404.html
----
-<div class="error-page">
-  <h1>404</h1>
-  <p>The page you are looking for does not exist or has been moved.</p>
-  <a class="back-link" href="/">← Back to infra</a>
-</div>

retro/404.html

@@ -1,13 +0,0 @@
----
-title: "Page Not Found"
-section: "retro"
-tags: "retro"
-description: "The page you are looking for does not exist."
-layout: base.njk
-permalink: /retro/404.html
----
-<div class="error-page">
-  <h1>404</h1>
-  <p>The page you are looking for does not exist or has been moved.</p>
-  <a class="back-link" href="/">← Back to retro</a>
-</div>

smarthome/404.html

@@ -1,13 +0,0 @@
----
-title: "Page Not Found"
-section: "smarthome"
-tags: "smarthome"
-description: "The page you are looking for does not exist."
-layout: base.njk
-permalink: /smarthome/404.html
----
-<div class="error-page">
-  <h1>404</h1>
-  <p>The page you are looking for does not exist or has been moved.</p>
-  <a class="back-link" href="/">← Back to smarthome</a>
-</div>