Add Cloud APIs to API Explorer

[lcawl]

This PR copies the current Cloud Billing, Cloud ECE, and Cloud Hosted API docs into this repo as a test of the docs in #2879

It proves that the tooling can handle Swagger (OpenAPI v2) documents. Open issues and questions:

1. How to handle situations where an OpenAPI document doesn't align with a single product ID (e.g. the Cloud Billing APIs) or one that doesn't exist in https://github.com/elastic/docs-builder/blob/main/config/products.yml (e.g. "cloud" or the "observability intake APIs")?
2. Poor tag names (I believe it's a known issue that this tool doesn't heed the x-displayName values or show the descriptions).

Steps to test

1. Run docs-builder serve -p /path/to/docs-builder/docs
2. Wait for messages like this to appear:

   info ::e.a.OpenApiGenerator  :: Generating OpenApiDocument Kibana APIs
   info ::e.a.OpenApiGenerator  :: Generating OpenApiDocument Elastic Cloud Serverless API
   info ::e.a.OpenApiGenerator  :: Generating OpenApiDocument Cloud Billing API
   info ::e.a.OpenApiGenerator  :: Generating OpenApiDocument Elastic Cloud API
   info ::e.a.OpenApiGenerator  :: Generating OpenApiDocument Elastic Cloud Enterprise API

3. Go to the appropriate URL (matches the keyword added in the docset.yml file). For example: http://localhost:3000/api/cloud-hosted/

[Mpdreamz]

Great validation! Good to know V2 OpenAPi works as intended in practice too.

For question 2, these will need to be exposed as products in that case (cloud-billing, manager-intake).

We should not embed OpenAPi in git, we should fetch them from our mirror. Some more work is needed to do so.

[lcawl]

For question 2, these will need to be exposed as products in that case (cloud-billing, manager-intake).

This was something I was pondering in the context of release notes as well -- do we need a "cloud" product ID or is it better to be able to apply multiple product Ids to a single item? I think in both the changelogs and APIs the latter is more accurate and clear (i.e. I think these billing APIs apply to both Cloud Serverless and Cloud Hosted but not Cloud Enterprise).