chore(docs): Clean up "What Are Gatsby Themes?" doc (#16462)
chore(docs): Clean up "What Are Gatsby Themes?" doc (#16462)

Minor grammar tweak:
- "But" -> "However,"

Sentence case article heading re: [style guide](https://www.gatsbyjs.org/contributing/gatsby-style-guide/#format-titles-and-headers)
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby