DOCS-206 - Deprecate Root Cause Explorer

[jpipkin1]

Purpose of this pull request

This pull request creates a deprecation article for Root Cause Explorer that will be published here:
https://help.sumologic.com/docs/observability/root-cause-explorer-deprecation

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

DOCS-206

[jpipkin1]

@jschwegler-sumo, could you review this PR?

[jpipkin1]

@jschwegler-sumo replied via Slack:

FAQ page looks great to me. I'll wait for Salvador's comments on the release note, not 100% sure about this statement "We believe many use cases for Root Cause Explorer can be met by other features in"
Once we are all in agreement, I think we can post the FAQ since it's not searchable, but hold the release note for now. I want to send another email internally that references the FAQ first and then also stage up the in-product with can coincide with the posting of the release note.

I replied in Slack:

I'll remove the release note from the PR. We'll publish the release note later.