[Logs Essentials] Update various references to project features/ECH to serverless comparisons throughout the docs

[mdbirnstiehl]

We should update the following pages to mention Logs Essentials and link to the new page with more information:

• - Project features.
• - Compare ECH to Serverless: add a note to the header that indicates the list is showing the full list of features for Observability Complete, and link to the new page for more information on Logs Essentials.
• - Observability billing dimensions: Add blurb about the different tiers and add Elastic Managed LLM information after Synthetics.

reference doc