Startup Box showing options for template, recent projects, and changelog

[ermannolocascio]

Fixes #2589

Progresses

• [x] Renaming the "Tutorials" tab to "Learning materials"

Include the following links with highlights

• [ ] "Hello world" equivalent: https://spine-toolbox.readthedocs.io/en/latest/getting_started.html
• [ ] Introduction to the spine data structure: https://github.com/energy-modelling-workbench/spine-data-model#spine-data-model
• [ ] Setting up a workflow: mention the section "Adding a Data Connection Item to the Project"
• [ ] executing projects: https://spine-toolbox.readthedocs.io/en/latest/executing_projects.html

Template tab

Checklist before merging

• [ ] Documentation is up-to-date
• [ ] Release notes have been updated
• [ ] Unit tests have been added/updated accordingly
• [ ] Code has been formatted by black
• [ ] Unit tests pass