Improve README with documentation index and clearer IO notes context

[Copilot]

The README did not describe the authored documentation, making the repo’s notes hard to navigate. This update adds a compact documentation index and clarifies the scope of referenced IO examples.

• Documentation overview

  • Added a top-level summary of the repository as a Haskell notes/practice workspace.
  • Introduced a dedicated section indexing the two written docs:
    • haskell_summary.md (core Haskell concepts and standard functions)
    • IO_Summary.md (IO return/effect behavior)

• Companion material mapping

  • Added a “Related practice files” section to connect .hs and data files to the written summaries.

• Terminology clarification

  • Updated wording to explicitly identify ff and fff as example functions, reducing ambiguity when scanning IO_Summary.md.

## Documentation overview

- `haskell_summary.md` — core Haskell topics and function/operator summaries
- `IO_Summary.md` — IO behavior notes using example functions `ff` and `fff`