…markdown transformer (payloadcms#16302)

## Overview

Payload has two types of links: custom (you type a URL) and internal
(you pick a page). For internal links, Payload never stores a URL, only
a reference to the page (`doc`). Whatever renders the link is
responsible for resolving that reference to a URL at render time.

The HTML converters handled this with an `internalDocToHref` callback.
The markdown transformer didn't, so it produced `[text](undefined)` for
every internal link.

## Key Changes

The transformer is now a `createLinkMarkdownTransformer(args?)` factory
that accepts an optional `internalDocToHref` callback. Pass it to
`LinkFeature` and it gets threaded through to the transformer. Without
it, internal links fall back to `[text]()` instead of
`[text](undefined)`.

`LinkMarkdownTransformer` remains as a static backwards-compatible
export.

## Design Decisions

The callback is synchronous unlike the HTML converter variant, because
`doc.value` is already populated by the time markdown export runs.

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