Specify subdirectory topic lead concepts

Inspired by https://github.com/enola-dev/wiki/commit/ccfd3f62d10c1b2605905b99d8c091fbd6d52a6a,

would it perhaps make sense to have the Spec spell out something along the lines of:

> If a directory contains a concept document named identically to the directory itself (e.g., `memory/memory.md`), it serves as the overarching topic concept. In auto-generated `index.md`, it is placed as the preamble immediately under the `# ` heading and omitted from `## Articles`.

Would a PR with a suggesting initial wording for this be welcome?

Start by reading the Spec and the referenced example commit, then inspect how a directory-named concept such as memory/memory.md relates to the generated index.md. Update the Spec with initial wording that defines the preamble placement under the category heading and exclusion from Articles; done means the behavior is unambiguous for implementers.

Written by the indexing model from the issue text.