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/workflows/ci.yml

@@ -6,6 +6,17 @@ on:
   pull_request:
 
 jobs:
+  check-pr-title:
+    name: Check PR Title
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          TITLE="${{ gitea.event.pull_request.title }}"
+          if ! echo "$TITLE" | grep -qE '^(WIP: )?(feat|fix|docs|chore|ci|test|refactor|perf|build|style|revert)(\(.+\))?(!)?: .+'; then
+            echo "::error::Pull Request title must follow conventional commits"
+            exit 1
+          fi
+
   lint-go:
     name: Go Lint
     runs-on: ubuntu-latest