Adds ELSER on EIS conceptual docs

[szabosteve]

Overview

This PR:

• adds initial docs about the Elastic Inference Service (EIS)
• adds docs about ELSER on EIS
• moves default inference endpoint list to the top of the Inference integrations page
• lists .elser-2-elastic under default inference endpoints
• links the EIS docs page on the Elastic Inference landing page
• changes inference to {{infer}}

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/elastic-inference/eis.md
• explore-analyze/elastic-inference.md
• explore-analyze/elastic-inference/inference-api.md
• explore-analyze/machine-learning/nlp/ml-nlp-elser.md

[leemthompo]

Looks great 👍!

One tiny aesthetic nit about the inline applies_tos tags: I think they look cleaner at the end of the line rather at the start, but definitely doesn't need to be addressed in this PR :)