docs: restructure README sponsor section and add pink fund button to docs

Fold the sponsor call-to-action into the Contributing section as a
natural bullet point instead of a standalone heavy section. Remove
the large badge buttons. Keep star history chart under Contributing.

Add a pink "Fund Development" button component to the docs site nav
bar (left of Star on GitHub), matching the landing page style.
Remove the plain-text nav link that replaced the heart icon.

apps/docs/.vitepress/config.mts

@@ -78,10 +78,6 @@ export default defineConfig({
       { text: "Home", link: "/" },
       { text: "Guide", link: "/guide/getting-started" },
       { text: "API Reference", link: "/api/rest" },
-      {
-        text: "Fund Development",
-        link: "https://github.com/sponsors/snapotter-hq",
-      },
       {
         text: `v${pkg.version}`,
         link: "https://github.com/snapotter-hq/snapotter/releases",