footer: add disclaimer

As per Minecraft's Usage Guidelines[1] a disclaimer must be provided on the
site to say that we are not associated with Minecraft, Mojang, or
Microsoft. Easiest and prettiest is on the site's footer.

[1]: https://www.minecraft.net/en-us/usage-guidelines#:~:text=Prominently,materials.

themes/minecraft/layouts/partials/footer.html

@@ -1,2 +1,5 @@
 <mcbe-theme-switch />
+<div class="ds-alert" data-color="info" data-size="sm">
+  NOT AN OFFICIAL MINECRAFT SERVICE. NOT APPROVED BY OR ASSOCIATED WITH MOJANG OR MICROSOFT
+</div>
 <p>{{ .Site.Copyright }}</p>