Basic guide for when to use different data types

As part of the documentation refresh work, add a basic user-facing guide for choosing among common Macaulay2 data types.

The goal is not full type-system documentation, but a practical guide that helps users answer questions like “which type should I use here?”

Possible scope:
- explain the most commonly confused container/data choices
- note which types are mutable vs immutable
- give short “use X when ..., use Y when ...” examples
- cross-link to the existing type documentation for deeper details

Examples that may be worth covering include:
- `List` vs `Sequence` vs `MutableList`
- `HashTable` vs `MutableHashTable`
- `Set` vs list-like collections
- other commonly encountered user-facing types if there are obvious gaps

This would be especially useful as an introductory node or short guide that can be linked from relevant documentation pages.

Start by locating the existing type documentation and the documentation navigation structure. Compare the commonly confused choices named in the issue, then draft a short user-facing guide with mutable/immutable notes, use-case examples, and links to deeper type documentation. Done means users can choose among the covered types without reading the full type-system reference.

Written by the indexing model from the issue text.