📘 DOC: Add Gatsby to Comparing section in docs

[kaushalyap]

Astro is compared to the lot of libraries here. But Gatsby is not mentioned there. AFAIK only difference is Gatsby tied to the React while Astro not. Astro claim to have partial hydration, Gatsby also eventually have partial hydration when React have Suspense. Other than that Gatsby have following features that I think Astro currently do not have.

• Image optimization
• Fast page navigation with Link
• Resource pre-fetching
• Per route code splitting
• Inlining Critical CSS
• Plugins

[jasikpark]

Gatsby is now in the comparison docs here: https://docs.astro.build/comparing-astro-vs-other-tools/