[E&A] Creates a page for guidance on when to use EIS and when to use ML nodes

[szabosteve]

Overview

Related issue: https://github.com/elastic/search-team/issues/11055 and #3008

This PR creates a page under the EIS section about when to use EIS and when to use ML nodes.

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/elastic-inference/ml-node-vs-eis.md

[florent-leborgne]

LGTM. Just a note:

The new page has the hidden attribute, which means that the page won't be published until we remove the hidden attribute.

It will actually be published. But it won't be indexed by search engines, nor appear in the navigation. Meaning it will be accessible with a direct URL only while it's hidden.

[szabosteve]

@florent-leborgne Thank you for your note. I updated the PR description accordingly.

[shubhaat]

We should try to not make this ML node v.s. EIS but instead have

1. Guidance on when to use ML nodes and when to use EIS
2. If you are using serverless search projects, or ECH and are already using an index using a model running on ML nodes, how do you use the model on EIS? -> the only way to do this is to create a new index, and index the data again.