feat: add metainfo.xml validation section to terra appstream page (#327)

* feat: add metainfo.xml validation section to terra appstream page

* oops

* revert "oops"

* please dont break this time

Author: Owen-sz

content/docs/terra/appstream.mdx

@@ -185,6 +185,20 @@ Source1: %appid.metainfo.xml   # This specifies your metainfo.xml file as a sour
 
 ---
 
+## Validating Your metainfo.xml File
+
+To validate your `metainfo.xml` file, install `appstream`, then run `appstreamcli validate NAME.metainfo.xml`.
+Errors and warnings should be corrected to the best of your ability. In rare cases, errors are fine to have, such as `E: com.github.neohtop:6: metainfo-invalid-icon-type local`.
+`infos` can be ignored or corrected at your discretion.
+
+Here is an example of the output of running `appstreamcli validate` on a proper metainfo.xml file:
+
+```bash
+appstreamcli validate games/minecraft-java/com.mojang.Minecraft.metainfo.xml
+I: com.mojang.Minecraft:50: nonstandard-gnome-extension metadata
+I: com.mojang.Minecraft:~: developer-info-missing
+``` 
+
 [^1]: libraries are also supported, see Freedesktop docs for tag provides: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-provides
 
 [^2]: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-keywords