Add a new tutorial for building a fitness app similar to Fitbud.
* **README.md**
- Add a new section "Build your own Fitness App".
- Add a link to the new tutorial for building a fitness app.
- Add a brief description of the fitness app tutorial.
* **ISSUE_TEMPLATE.md**
- Add a new category "Fitness App" to the list of categories.
* **tutorials/fitness-app.md**
- Create a new file for the fitness app tutorial.
- Add a step-by-step guide for building a fitness app similar to Fitbud.
- Include code snippets and explanations for each step.
- Add links to relevant resources and documentation.
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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/codecrafters-io/build-your-own-x?shareId=XXXX-XXXX-XXXX-XXXX).
