[DOCS] Add placeholders for Observability docs

[EamonnTP]

This PR adds content placeholders for the forthcoming Observability docs. This directory and the files will be included in the migration of the Logs and Metrics directories to the new Observability repo.

The following PRs need to be merged before migrating the Logs and Metrics directories:

#1210
#1203

Related PRs:

elastic/docs#1877

[dedemorton]

I know this is all placeholder stuff, but we should start thinking about how we want to structure the content.

1 - We definitely want a one to one mapping between topics and HTML output. So one asciidoc file = one HTML file.

2 - To control TOC nesting, we should explore using level offsets. I've done this in the ingest management docs with mixed success. (The strategy worked for +1 and +2 offsets, but broke for anything nested more deeply.) We should follow up with other teams to see if they've had more success, but I think it's the best way forward to make it easier to decouple TOC/navigation from heading levels.

[bmorelli25]

Neat! I agree with DeDe that we should start having conversations about a consistent structure for the Observability content.

I'm say we merge this as-is and update the structure when the actual content is added to this book.

[bmorelli25]

Small note: I'd also like to see us remove the unnecessary /en/ directory in our docs.

Edit: Maybe even /docs/en/? Something else to chat about.

[dedemorton]

Let's bring up the question about /docs/en/ with other writers in the org. Eventually we will need language-specific folders for translation.

[bmorelli25]

Ahh, I didn't realize that was still something we were planning on doing.