fix(docs-template): type guard for boolean

isPageIncomplete variable is truthy or falsy but the banner notification
component accept a boolean props. So we convert truthy or falsy to true
or false

Refs: ethereum#6392

src/templates/docs.tsx

@@ -178,7 +178,7 @@ const DocsPage = ({
   const isRightToLeft = isLangRightToLeft(mdx.frontmatter.lang as Lang)
 
   const tocItems = mdx.tableOfContents?.items
-  const isPageIncomplete = mdx.frontmatter.incomplete
+  const isPageIncomplete = !!mdx.frontmatter.incomplete
 
   const { editContentUrl } = siteData.siteMetadata || {}
   const { relativePath, slug } = pageContext