[docs-beta] add contributing migration section (dagster-io#26483)

## Summary & Motivation

- Adds some pointers on migrating between legacy and new docs

## How I Tested These Changes

## Changelog

> Insert changelog entry or delete this section.

docs/docs-beta/CONTRIBUTING.md

@@ -0,0 +1,100 @@
+# Contributing
+
+## Migration from legacy docs
+
+There are some features in the previous docs that require changes to be made to work in the new Docusaurus-based documentation site.
+
+### Images
+
+Before:
+
+```
+<Image
+  alt="Highlighted Redeploy option in the dropdown menu next to a code location in Dagster+"
+  src="/images/dagster-cloud/developing-testing/code-locations/redeploy-code-location.png"
+  width={1920}
+  height={284}
+/>
+```
+
+After:
+
+```
+<ThemedImage
+  alt="Highlighted Redeploy option in the dropdown menu next to a code location in Dagster+"
+  style={{width:'100%', height: 'auto'}}
+  sources={{
+    light: '/images/dagster-cloud/developing-testing/code-locations/redeploy-code-location.png',
+    dark: '/images/dagster-cloud/developing-testing/code-locations/redeploy-code-location.png',
+  }}
+/>
+```
+
+### Notes
+
+Before:
+
+```
+<Note>This guide is applicable to Dagster+.</Note>
+```
+
+After:
+
+```
+:::note
+This guide is applicable to Dagster+
+:::
+```
+
+### Tabs
+
+Before:
+
+```
+<TabGroup>
+  <TabItem name="Amazon ECS">
+  ...
+  </TabItem>
+</TabGroup>
+```
+
+After:
+
+```
+<Tabs>
+  <TabItem value="Amazon ECS">
+  ...
+  </TabItem>
+</Tabs>
+```
+
+### Header boundaries
+
+Previously, horizontal rules had to be defined between each level-two header: `---`.
+
+This is no longer required, as the horizontal rule has been included in the CSS rules.
+
+### Reference tables
+
+Before:
+
+```
+<ReferenceTable>
+  <ReferenceTableItem propertyName="container_context.ecs.env_vars">
+    A list of keys or key-value pairs to include in the task. If a value is not
+    specified, the value will be pulled from the agent task.
+    <br />
+    In the example above, <code>FOO_ENV_VAR</code> will be set to{" "}
+    <code>foo_value</code> and <code>BAR_ENV_VAR</code> will be set to whatever
+    value it has in the agent task.
+  </ReferenceTableItem>
+</ReferenceTable>
+```
+
+After:
+
+_There is not a replacement at this point in time..._
+
+### Whitespace via `{" "}`
+
+Forcing empty space using the `{" "}` interpolation is not supported, and must be removed.