Document EIS/Inference billing on Observability and Security Serverless billing dimensions

## Summary

Elastic Inference Service (EIS) token billing appears to be documented more explicitly on the [Elasticsearch Serverless billing dimensions](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/elasticsearch-billing-dimensions) page (as **Tokens**) than on the [Observability](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/elastic-observability-billing-dimensions) or [Security](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/security-billing-dimensions) Serverless billing dimensions pages, which currently focus on Elastic Managed LLMs token charges.

HOWEVER, there is a "shared" EIS section on the pricing table (appears on sec/obs tabs), which means that there is some sort of bug here. https://cloud.elastic.co/cloud-pricing-table?productType=serverless&group=serverless-projects&solution=security&provider=aws®ion=us-east-1

Found while addressing a semi-related bug: https://github.com/elastic/docs-content/issues/7609 / https://github.com/elastic/docs-content/pull/7938.

## Investigate

Confirm whether EIS/Inference billing works the same way across Elasticsearch, Observability, and Security serverless projects (pricing table, Usage dimension, and product behavior), relationship to + positioning with elastic-managed LLMs

SR: I started this, but I do not understand the relationship between EIS and elastic-managed LLMs, so I'm deferring to experts on dev/ski for positioning.

## If consistent across products

Document it a level up — on the [Serverless billing dimensions](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/serverless-project-billing-dimensions) hub — rather than repeating or partially restating it on each project-type page. Keep project pages for offering-specific notes only if needed (including how Managed LLMs relate, if that remains a useful distinction).

## Type

Missing information / consistency — investigate first

Start with the Elasticsearch, Observability, and Security Serverless billing dimensions pages and the linked cloud pricing table. Compare EIS/Inference billing across pricing, Usage dimensions, and product behavior, including its relationship to Elastic Managed LLMs; consult the relevant experts for positioning. Done means the consistent guidance is documented at the Serverless billing dimensions hub, with project pages retaining only necessary offering-specific notes.

Written by the indexing model from the issue text.