docs(gatsby): html.js docs for themes (#19211)
docs(gatsby): html.js docs for themes (#19211)

* docs(gatsby): html.js docs for themes

#19181

* docs(html): html.js custom placement

* docs(htm): Update docs/docs/custom-html.md

Co-Authored-By: Marcy Sutton <63f38fb7a2c9e72439a06d93cefb66621e8fc496@gatsbyjs.com>

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