feat(doc): update structure

[Yordan-Ramchev]

re #368

Updated:

1. Homepage - links based on new structure
2. Header navigation - Guide link (to match Router and Pinia doc), playground link & GitHub link
3. Getting Started - contains Introduction, Features & Installation pages
4. Moved FAQ & Contribution to a Help section
5. Added some info texts

Nice to have: https://github.com/vuejs/create-vue-templates/tree/069649e4613d3430f930935e2dbdb76a6a80ab2e/devtools — In order to view pinia and router integration, we may need to update playground template to devtools-router-pinia

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[webfansplz]

Great work! Thank you~