docs(themes): Make naming convention required and not recommended (#16620)
docs(themes): Make naming convention required and not recommended (#16620)

* Edited the naming convention rule for gatsby theme

Prefixing the package with "gatsby-theme" is required (not recommended).

* Update docs/docs/themes/conventions.md

Co-Authored-By: Sidhartha Chatterjee <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@sidharthachatterjee.com>

gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby