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/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,41 @@
+<!--
+Thanks for contributing to Calendula!
+
+Please skim CONTRIBUTING.md if you haven't:
+https://codeberg.org/jlmakiola/calendula/src/branch/main/CONTRIBUTING.md
+
+Two things it's easy to get wrong:
+  • Features need a discussed issue first — an undiscussed feature PR may be
+    closed unmerged even when the code is good.
+  • Target the release branch for your issue's milestone (milestone 2.18.0 →
+    release/v2.18.0), not main. If you targeted main, just say so below and it
+    will be retargeted.
+-->
+
+### What this changes
+
+
+### Why
+
+<!-- Closes #123 — link the issue this implements or fixes. -->
+
+
+### How it was tested
+
+<!--
+Which of these ran green, and anything you exercised by hand. On-device notes
+are especially useful for UI changes.
+
+  ./gradlew lint test assembleDebug
+  python3 scripts/check_translations.py
+-->
+
+
+### Checklist
+
+- [ ] There's an issue for this, and (for a feature) it got a go-ahead
+- [ ] Targeting the release branch for that issue's milestone — or `main`, noted above
+- [ ] `./gradlew lint test assembleDebug` passes locally
+- [ ] No `values-*/strings.xml` touched (Weblate owns those; new English strings in `values/` are fine)
+- [ ] `CHANGELOG.md` updated under `## [Unreleased]`, if the change is user-visible
+- [ ] No planning or design documents committed