Docs: point the RossoCortex repository README link at README.md

[mrsabath]

Summary

The "Install it" section of the RossoCortex concept page linked the repository
README to https://github.com/rossoctl/cortex/blob/main/index.md. That file does
not exist in the rossoctl/cortex repository — the root holds README.md — so the
link rendered a GitHub 404 on the published page:
https://www.rossoctl.dev/docs/concepts/core/cortex/#install-it

This points the link at README.md.

Single-line change in versioned_docs/version-0.8/concepts/core/cortex.md:

-[repository README](https://github.com/rossoctl/cortex/blob/main/index.md)
+[repository README](https://github.com/rossoctl/cortex/blob/main/README.md)

Notes on scope:

• This was the only occurrence of the broken link in the repository; no other
  blob/main/index.md references exist.
• There is no unversioned docs/ directory here, so version-0.8 is the only
  source for this page and the fix will not be undone by a later version cut.
• The v0.7 copy of the page does not contain the link, so it needs no change.

Related issue(s)

None.

(Optional) Testing Instructions

Confirm the target exists and the old one does not:

curl -sI https://github.com/rossoctl/cortex/blob/main/README.md | head -1   # 200
curl -sI https://github.com/rossoctl/cortex/blob/main/index.md  | head -1   # 404

Then build the site and follow the "repository README" link in the "Install it"
section of Concepts → Core → RossoCortex.

Assisted-By: Claude Code

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