Add architecture overview

architecture.md

+# Architecture overview
+
+1. Webserver takes and validates user submitted files
+2. Cron job scans files and enqueues new jobs on cluster
+3. Job gets processed on the cluster using [whisper-webvtt-transcriber](https://gitlab1.ptb.de/janhartig/whisper-webvtt-transcriber)
+4. Mailservice scans job folders for completed jobs and:
+   - Sends processed files to users
+   - Optional: Notifies admins on processing errors
+   - Optional: Sends monitoring data to webserver 
+
+
+State is tracked through the following files in the jobs folder:
+   - new: Job has been submitted by user
+   - done: Job has been processed without errors
+   - error: Job has been processed with errors
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