Make JSDoc for API files translateable

[tesseralis]

Summary

Much of the content of the API section of the website is directly sourced from documentation through Documentation.js. This content can't be as easily extracted for translation as markdown files or yaml strings, so we need to figure out a way to deal with them.

Steps

• Convert API docs pages to MDX Convert API pages to MDX. #19035
• Make API JSDocs more easily extractable for translation Make JSDoc API files more easily extractable for translation #19383
• Figure out what to do with actions API and node-model Figure out what to do with the node-model and actions API JSDoc. #19382

[tesseralis]

Doing research, it seems like most of the API is documented in individual files:

• https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/utils/api-node-docs.js
• https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/utils/api-browser-docs.js

The only one that isn't is node-model: https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/schema/node-model.js

Given this, the process should be something like

• put these files in their own dedicated folder instead of just utils
• factor out "api-node-model-docs"
• extract these files into gatsby-i18n-source

[muescha]

• factor out "api-node-model-docs" -> Move node-model documentation to utils #19027