Fix `description` fields in content

[delucis]

Several pages in src/content/docs/ need the description field in their frontmatter updating to reflect what each page is about:

• https://github.com/volarjs/docs/blob/main/src/content/docs/index.mdx
• https://github.com/volarjs/docs/blob/main/src/content/docs/reference/services.md
• https://github.com/volarjs/docs/blob/main/src/content/docs/guides/first-server.mdx
• https://github.com/volarjs/docs/blob/main/src/content/docs/core-concepts/why-volar.mdx
• https://github.com/volarjs/docs/blob/main/src/content/docs/core-concepts/embedded-languages.mdx