Use CONFIG.SITE.title for h1 in doc theme header (#2449)

Use SITE.title specified in config.ts as the h1 heading in Header.astro
of doc theme.
When SITE.title is null, use "Documentation" as the fallback value.
Previously the h1 heading in doc theme header is hardcoded
as "Documentation".

examples/docs/src/components/Header/Header.astro

@@ -20,7 +20,7 @@ const lang = currentPage && getLanguageFromURL(currentPage);
 		<div class="logo flex">
 			<AstroLogo size={40} />
 			<a href="/">
-				<h1>Documentation</h1>
+				<h1>{CONFIG.SITE.title ?? "Documentation"}</h1>
 			</a>
 		</div>
 		<div style="flex-grow: 1;"></div>