docs: improvements to railway deployment guide + new troubleshooting guide (#7012)

www/apps/docs/content/troubleshooting/mime-error.md

@@ -0,0 +1,17 @@
+---
+title: "Mime Error"
+---
+
+The following error can occur when running the `build` command either locally or in an deployed Medusa backend:
+
+```
+Cannot find type definition file for 'mime'
+```
+
+This error typically occurs when using yarn v1 in an older Medusa setup.
+
+To resolve the error, install v1.3.5 of `@types/mime` as a development dependency in your Medusa backend:
+
+```bash npm2yarn
+npm install @types/mime@1.3.5 --save-dev
+```