docs: update general graphcast docs, add a page for 3la radio

[hopeyen]

Update sections

• Design principle
• Graphcast intro
• SDK intro - a few subsections added
• POI new user configs
• 3LA page under the radio category

[neriumrevolta]

Amazing work 🚀 just some formatting & rewording suggestions

[hopeyen]

thanks for the review. I resolved some of the suggestions but left some personal preferences on the docs (mostly on the capitalization of phrases); personally would rather not make those commit changes, but feel free to do so when you update the docs. will squash before merge

[neriumrevolta]

thanks for the review. I resolved some of the suggestions but left some personal preferences on the docs (mostly on the capitalization of phrases); personally would rather not make those commit changes, but feel free to do so when you update the docs. will squash before merge

sgtm