docs: [#12] add MySQL migration implementation plan

- Add comprehensive implementation plan for migrating from SQLite to MySQL
- Include correct MySQL connection string format (path parameter)
- Document automatic database migrations by Torrust Tracker
- Add local Docker Compose testing strategy before VM testing
- Update docs structure to include issues directory
- Fix configuration based on actual Torrust Tracker MySQL driver

The plan follows Phase 1 of the Hetzner migration strategy and provides
step-by-step implementation guide with proper testing methodology.

Author: josecelano

docs/README.md

@@ -26,6 +26,13 @@ This directory currently contains cross-cutting documentation:
 - [Hetzner Migration Plan](plans/hetzner-migration-plan.md) - Comprehensive plan
   for migrating from Digital Ocean to Hetzner infrastructure
 
+### 🎯 [`issues/`](issues/) (Implementation Plans)
+
+**Issue Implementation Plans:**
+
+- [Phase 1: MySQL Migration](issues/12-use-mysql-instead-of-sqlite-by-default.md) -
+  Detailed implementation plan for database migration from SQLite to MySQL
+
 ### Future Categories
 
 The following directories can be created as needed: