Ignore unknown vendor extensions

Is your feature request related to a problem?

My API contains custom vendor extensions so it does certain actions as part of the build. The plugin should ignore or skip any it doesn't know.
(I wasn't sure if this was a bug or enhancemenmt)

For example:

"x-coa-command-group": "nodes",

I have to remove them each time, otherwise I get an error like:

> docusaurus gen-api-docs all

Loading of api failed for "/Users/tatiana/Documents/coalesce-docs/static/api/coalesce-api.json"

[ERROR] TypeError: Cannot create property 'description' on string 'nodes'
    at createItems (/Users/tatiana/Documents/coalesce-docs/node_modules/docusaurus-plugin-openapi-docs/lib/openapi/openapi.js:109:45)
    at processOpenapiFile (/Users/tatiana/Documents/coalesce-docs/node_modules/docusaurus-plugin-openapi-docs/lib/openapi/openapi.js:511:19)
    at async /Users/tatiana/Documents/coalesce-docs/node_modules/docusaurus-plugin-openapi-docs/lib/openapi/openapi.js:464:35
    at async Promise.all (index 0)
    at async processOpenapiFiles (/Users/tatiana/Documents/coalesce-docs/node_modules/docusaurus-plugin-openapi-docs/lib/openapi/openapi.js:475:22)
    at async generateApiDocs (/Users/tatiana/Documents/coalesce-docs/node_modules/docusaurus-plugin-openapi-docs/lib/index.js:95:44)
    at async /Users/tatiana/Documents/coalesce-docs/node_modules/docusaurus-plugin-openapi-docs/lib/index.js:439:15
[INFO] Docusaurus version: 3.4.0

Describe the solution you'd like

Ignore the unknown vendor extensions. Don't try to process or do anything with them.

Describe alternatives you've considered

None, I need them for code generation for the company app. I can't take them out.

Additional context

If left in, I get error messages and the reference won't generate.

Start with lib/openapi/openapi.js, especially createItems and processOpenapiFile, using the docusaurus gen-api-docs all reproduction. Verify that x-coa-command-group and other unknown x-* values no longer fail generation while standard OpenAPI fields are still processed. The payload does not name a test file, so locate the relevant coverage before validating the change.