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/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,2 @@
+# yaml-language-server: $schema=https://www.schemastore.org/gitea-issue-config.json
+blank_issues_enabled: true