refactor(docs): index: add badge to point to latest tag version

docs/index.md

+[![latest version](https://img.shields.io/badge/dynamic/json?color=green&label=latest%20release&query=%24%5B0%5D.name&url=https%3A%2F%2Ftransfer.hft-stuttgart.de%2Fgitlab%2Fapi%2Fv4%2Fprojects%2F154%2Frepository%2Ftags%3Fper_page%3D1)](https://transfer.hft-stuttgart.de/gitlab/9Lukas5/unified-ticketing/-/packages)
 [![pipeline state](https://transfer.hft-stuttgart.de/gitlab/9Lukas5/unified-ticketing/badges/master/pipeline.svg)](https://transfer.hft-stuttgart.de/gitlab/9Lukas5/unified-ticketing/pipelines)
 [![License GPLv3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 [![Conventional Commits](https://img.shields.io/badge/Conventional%20Commits-1.0.0-orange.svg)](https://conventionalcommits.org)