Observability documentation init

[bmorelli25]

Summary

This PR sets up the observability-docs repo:

• Removes stack-docs content
• Removes the /en/ directory and nests documentation content under docs/
• Updates the readme to something basic
• Updates backportrc.json

The following content remains:

The big picture

1. Merge this PR to update the observability-docs repo.
2. Add this content to the documentation build by updating and merging [WIP] Add Observability docs docs#1877.
3. Delete the original content in stack-docs by merging Move observability docs stack-docs#1233.

[bmorelli25]

cla/check

[bmorelli25]

I've opened elastic/stack-docs#1233 to remove this content from stack-docs.

[dedemorton]

LGTM