docs: link common types to shared references section (#536)

* docs(specification): centralize shared types per reference.md (#412)

PR #226 made `reference.md` the canonical render destination for all
`types/` schemas (cross-references already resolve there via the
redirect logic in `main.py`). However, capability pages still
re-rendered the same `types/...` field tables inline, leaving three
copies of each shared type (cart.md, checkout.md, catalog/index.md)
alongside the canonical render in reference.md.

This change finishes the centralization:

* `schema_fields(entity, spec)` and `extension_schema_fields(entity,
  spec)` now detect when the requested schema lives under a `types/`
  directory (common or vertical) and, for any caller other than
  `reference.md`, emit a Markdown link to the canonical entry instead
  of an inline table.
* `reference.md` continues to render the full tables via
  `auto_generate_schema_reference`, so the source of truth is
  unchanged.
* Existing capability-page macro calls (`{{ schema_fields('types/
  signals', 'cart') }}`, etc.) work unmodified; the surrounding prose
  context on each capability page is preserved.

The macro change is intentionally minimal: no `.md` files are touched,
keeping the diff narrow and reversible.

Out of scope (follow-ups):

* wry-ry's suggestion in the issue thread to replace the "Required"
  column with a "Visibility" column (`required` / `optional` / `omit`)
  is deferred — it needs an `omit` annotation in the schemas which
  doesn't exist today. Worth a separate issue once the data model is
  agreed.

* scope shared-type consolidation to common/types/

   Refines the consolidation pattern introduced for #412. The first pass
   redirected every `types/...` reference -- including vertical-namespaced
   types like Buyer, Total, Line Item, and Fulfillment Option -- to
   reference.md, converting capability pages into runs of "See X in the
   Schema Reference" sentences and dropping the operation-filtered variant
   tables (Create Request vs. Update Request vs. Response) that integrators
   rely on when reading a capability end-to-end.

   This commit narrows the redirect to schemas under `common/types/` only.
   Cross-vertical primitives -- Pagination, Error Code, Postal Address,
   Amount, Link, Media, Message and its variants -- still consolidate to
   reference.md, because their fields are not informative inline on a
   capability page. Vertical-namespaced types under `<vertical>/types/`
   retain their inline render so the variant logic in `_resolve_schema`
   keeps producing the correct table for each section.

   Render duplication of vertical types across capability pages is
   intentional and safe: every render is generated from the same JSON
   source via ucp-schema, so drift between them is structurally impossible.

   The redirect rule in `create_link` (introduced in #226) is unchanged,
   so cross-references emitted from inside any rendered table still target
   reference.md and anchor consistency is preserved end-to-end.

   Link-sentence count per capability page:

       page                full-consolidation   common-only
       ----                ------------------   -----------
       cart                                14             5
       checkout                            21             8
       catalog/index                       15             5
       fulfillment                         12             1
       order                                7             1
       embedded-checkout                    6             1

   Common-only retains link sentences only for the cross-vertical
   primitives each page actually references:

       page                redirected to reference.md
       ----                --------------------------
       cart                Link, Message,
                           Message Error / Info / Warning
       checkout            Error Code, Error Response, Link,
                           Message, Message Error / Info / Warning,
                           Postal Address
       catalog/search      Pagination Request / Response  *
       fulfillment         Postal Address
       order               Postal Address
       embedded-checkout   Postal Address

       * via extension_schema_fields, emitted as a bare cross-ref
         rather than a "See X in..." sentence

* drop type-name enumerations from rationale

This commit replaces every type-name list in the docstrings and
inline comments introduced by the previous commit with structural
descriptions (redirect predicate by path, polymorphic-variant
mechanic) and genericized URL shapes. The collision-impossibility
clause in `_resolves_to_shared_type` is rephrased as a stated
assumption rather than a snapshot of the current `common/` directory.

Functional behavior and rendered output are unchanged.
mkdocs build clean; check_links.py validates all internal links.