[codex] Move config reference into configs section

[abegong]

Summary

• Move the Reference "Configuration" page into a new reference/configs/ section as Configs.
• Split the config reference by concept: discovery, schemas, bases, collections, checks, variants, and listing.
• Update internal links to the new concept pages and preserve the old /reference/configuration/ URL as an alias.

Organization

docs/content/reference/configs/ is now organized as:

• _index.md: overview, .katalyst/ layout, and a map to the concept pages.
• discovery.md: root discovery, symlink handling, file layout, formats, and legacy storage paths.
• schemas.md: schema discovery, naming, and schema handles.
• bases.md: backend stores and top-level base keys.
• collections.md: collection membership, identity, paths, attributes, content, and per-collection files.
• checks.md: schema: shorthand, checks: entries, text rules, and object-schema precedence.
• variants.md: conditional check routing with when.
• listing.md: project and collection defaults for katalyst item list.

Validation

• hugo --source docs --destination /private/tmp/katalyst-docs-build-approach2

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