[docs] fix docusaurus due to previous upgrade being incompatible (#2710)

## Summary

Docusaurus was failing to start due to an incompatible upgrade in #2684:
`Module not found: Error: Can't resolve '@theme/ResponseSamples'`

Also updated README to reflect new tooling. 
Finally, I tried to fix the local `pnpm start` opening to
`localhost:3000/docs/devbox` but didn't succeed for now. Left a note in
the README to alert developers about it.

**Upgrade Fix**
- The component name changed from ResponseSamples to ResponseExamples,
but more importantly, the component API also changed:
- Old (v4.4.0):
```
  import ResponseSamples from "@theme/ResponseSamples";

  <ResponseSamples
    responseExample={"{\n  \"query\": \"string\", ...}"}
    language={"json"}
  />
```
- New (v4.5.1):
The new version uses a completely different component structure - it no
longer uses ResponseSamples or ResponseExamples directly in the
generated MDX. Instead, it uses a different set of components like
StatusCodes, RequestSchema, etc.


```
# Regenerate the API docs from the OpenAPI spec
pnpm docusaurus gen-api-docs nixhub

# Move the regenerated files to the correct location
cp docs/nixhub/*.api.mdx nixhub/

# Clean up the temporary directory
rm -rf docs/nixhub
```

Files Updated

- nixhub/search-packages.api.mdx
- nixhub/resolve-a-package-version.api.mdx
- nixhub/get-a-package.api.mdx

These files now use the updated component structure compatible with
docusaurus-theme-openapi-docs v4.5.1.

## How was it tested?

`devbox run start` and opened devbox docs as well as the Nixhub docs.

`devbox run build` and `devbox run serve` and opened the docs again in
localhost, just to be safe and ensure the production environment will
work in Vercel.

## Community Contribution License

All community contributions in this pull request are licensed to the
project
maintainers under the terms of the
[Apache 2 License](https://www.apache.org/licenses/LICENSE-2.0).

By creating this pull request, I represent that I have the right to
license the
contributions to the project maintainers under the Apache 2 License as
stated in
the
[Community Contribution
License](https://github.com/jetify-com/opensource/blob/main/CONTRIBUTING.md#community-contribution-license).

Author: savil

docs/README.md

@@ -8,21 +8,22 @@ This website is built using [Docusaurus](https://docusaurus.io/), a modern stati
 ```bash
 cd docs/app     # from the devbox root directory
 devbox shell    # optional, develop inside a devbox
-yarn install    # run in devbox shell
+pnpm install    # run in devbox shell
 ```
 
 ### Local Development
 
 ```bash
-yarn start
+devbox run start
 ```
 
 This command starts a local development server and opens up a browser window. Most changes are reflected live without having to restart the server.
+NOTE: this will open localhost:3000/docs that will show a 404 Not Found. Go to localhost:3000/docs/devbox instead to find the real docs.
 
 ### Build
 
 ```bash
-yarn build
+devbox run build
 ```
 
 This command generates static content into the `build` directory and can be served using any static contents hosting service.