chore: Port FreeSewing.dev to docusaurus

The replaces the NextJS site powering FreeSewing.dev with a Docusaurus
setup. It's part of my efforts to simplify FreeSewing's setup so we can
focus on our core value proposition.

sites/dev/docs/reference/api/utils/mergei18n/readme.mdx

@@ -0,0 +1,84 @@
+---
+title: utils.mergeI18n()
+---
+
+The `utils.mergeI18n()` function merges translation files from different designs.
+
+When your design re-uses parts from other designs, it should provide
+translation for those parts and any strings or options they use.  Rather than
+re-creating this content in your own design, you can pull it out of of the
+design you are using the part from by importing the `i18n` named export, and
+then use this function to merge it with your own translations.
+
+## Signature
+
+```js
+Object mergeI18n({
+  Array designs,
+  Object options
+})
+```
+
+## Use
+
+In its simplest form, this function will receive an Array of translation data
+(typically the `i18n` named export of a design) and merge them:
+
+```js
+import { i18n as brianI18n } from '@freesewing/brian'
+import { i18n as simonI18n } from '../i18n/index.mjs'
+
+// Merge translations
+const i18n = mergeI18n([brianI18n, simonI18n])
+```
+
+This simple approach is typically what you want when you are extending a design
+and you are keeping all the parts and options of the design you are extending.
+In the example above, Simon extends Brian in this way.
+
+## Configuration
+
+If you don't want to keep all options or parts, you can further control how the
+translation data will be merged by passing in a second parameter which is an
+object holding the configuration that describes how to merge the translation
+data.
+
+The configuration object takes 3 top-level properties:
+
+- `s`: Specifies configuration for how to merge the translation of strings (under `s` in the translation files)
+- `p`: Specifies configuration for how to merge the part name translations (under `p` in the translation files)
+- `o`: Specifies configuration for how to merge the option translations (under `p` in the translation files)
+
+For **each of the `s`, `p`, and `o` keys** you can specify the following properties:
+
+- `drop`: An Array with the keys of entries to not merge (drop). Keys that are not in this array will be merged.
+- `keep`: An Array with the keys of entries to merge (keep). Keys that are not in this array will not be merged.
+
+### Example
+
+```js
+import { i18n as brianI18n } from '@freesewing/brian'
+import { i18n as otherI18n } from '../i18n/index.mjs'
+
+// Merge translations
+const i18n = mergeI18n(
+  [brianI18n, otherI18n],
+  {
+    p: {
+      keep: ['sleevecap'],
+    },
+    o: {
+      drop: ['waistEase', 'chestEase']
+      }
+    }
+  }
+)
+```
+
+The function will check each key under the `s`, `p`, and `o` top-level keys in the translation files.
+For each it will:
+
+- Check whether there is a `drop` array configured. If there is, it will remove the entry if its key is included in the `drop` Array.
+- Check whether there is a `keep` array configured. If there is, it will remove the entry if its key is not included in the `keep` Array.
+
+If the entry is not explicitly removed by including it in `drop` or excluding it from a list set in `keep` it will be kept.