Add tutorial for building a fitness app

[scarruth1872]

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.

---

For more details, open the Copilot Workspace session.