Doc: Replace missing intro file for eval/edit

[karenzone]

A source file with introductory content was removed from the Logstash Reference in error during a major overhaul. This PR:

• re-adds the file
• Moves additional introductory content out of index file and to the re-added file where it makes more sense. (The "other" introductory content floating loose in the index file is likely what led to this error.)
• Fixed some headings to bring them up to standard wrt sentence capitalization (because I can't help myself)

Objectives

Evaluate this content to see if we want to keep it. If so, update it to bring it inline with current product status/messaging and doc standards.

Related: #14216. I discovered this issue when I was reworking introductory content in #14220.

Note to self @karenzone: If we move forward with this PR, #14220 will need some rework (and conflict resolution) to remove duplicate changes.

Preview: https://logstash_14221.docs-preview.app.elstc.co/guide/en/logstash/master/introduction.html

[github-actions[bot]]

:page_with_curl: DOCS PREVIEW :sparkles: https://logstash_14221.docs-preview.app.elstc.co/diff

[karenzone]

@elasticmachine run elasticsearch-ci/docs

[karenzone]

@flexitrev, here's the PR we discussed yesterday. I'm looking forward to working with you on this.