feat: Added Sanity documentation

markdown/dev/reference/sites/sanity/en.md

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+---
+title: Sanity
+---
+
+FreeSewing uses [Sanity](https://www.sanity.io/) -- a headless CMS, or a SaaS
+platform for structure content -- to host various types of content.
+
+For background information, please refer to [the Sanity content
+guide](/guides/content/sanity).
+
+## Content schema
+
+The Sanity content scheme is stored in `sites/sanity/schema` in our monorepo.
+
+## Sanity studio
+
+The `sites/sanity` folder holds an instance of Sanity Studio -- the frontend to
+manage the content -- preconfigured to work with our content.  This site is
+published at https://cms.freesewing.org/ where editors can work on our content.
+
+### Authentication
+
+To be able to use the Sanity Studio with FreeSewing's data, you need to be
+added as an editor.  Sanity supports using your GitHub account to authenticate,
+so if you want to become an editor, you can ask joost to grant you access.
+
+### Local development
+
+After setting up the monorepo with `yarn kickstart` in the root folder, change your working directoy to `sites/sanity` and run `yarn dev`:
+
+```sh
+git clone git@github.com:freesewing/freesewing.git
+cd freesewing
+yarn kickstart
+cd sites/sanity
+yarn dev
+```
+
+The Sanity Studio UI will open at http://localhost:3333
+
+<Warning compact>
+This instance is setup to work with our production data. 
+</Warning>
+
+## Sanity API
+
+
+The following data is required to interact with Sanity:
+
+- Project ID: `hl5bw8cj`
+- Dataset: `production`
+
+With that and [the API reference
+documentation](https://www.sanity.io/docs/reference) you should be able to get
+started.
+