Write a contributing guide, and fix what the forge move left stale (#96)

(docs) Update all the docs to be up to date with current status quo

.forgejo/ISSUE_TEMPLATE/crash_report.md

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+---
+name: Crash report
+about: Report a crash. Calendula can capture this for you (Settings → Report a problem, or the prompt after a crash) — it copies the report to your clipboard and prefills this form.
+title: "Crash: "
+labels:
+  - bug
+  - crash
+  - priority/high
+---
+
+<!--
+Thanks for reporting a crash in Calendula!
+
+If the app prefilled this for you, the crash report is already below — just add
+what you were doing and submit. Otherwise, paste the report from your clipboard
+into the code block. The report contains only app/Android/device versions and the
+stack trace — no personal data or calendar content.
+-->
+
+### What happened
+
+
+### Crash report
+
+```
+(paste the crash report here)
+```