ci: support semantic versioning, conventional commits (#15)

## Description

Currently, the repository is not well suited for semantic versioning, and hasn't much to support it. This PR adds templates, CI jobs, and configs to simplify its adoption.

## Changes

- Added `FEATURE` and `BUG` issue templates. Also, forbids free-form issues.
- Adds a PR template.
- Adds a CI job to ensure the commit title follows conventional commits.

### Design Decisions

N/A.

## Checklist

- [x] Tests pass
- [x] Docs updated

Reviewed-on: #15
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

.gitea/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,17 @@
+---
+name: "New Pull Request"
+about: "Standard PR template"
+title: ""
+ref: "main"
+---
+
+## Description
+
+## Changes
+
+### Design Decisions
+
+## Checklist
+
+- [ ] Tests pass
+- [ ] Docs updated