Support different formats in alternate links

Problem/Motivation

The <link rel="alternate" type="text/markdown"> meta tag added by markdownify_page_attachments() always uses the .md extension format (e.g. /node/1.md).
This URL format may not work in all server environments — for example, certain nginx configurations block or do not pass through requests ending in .md.
The module itself already supports multiple URL formats to access Markdown content (.md extension, _format=markdown query parameter, and /markdownify/ prefix routes), but there is no way to control which format is used in the advertised alternate link.

Steps to reproduce

1. Install and configure the markdownify module.
2. Visit a supported entity page (e.g. a node).
3. Inspect the HTML - the <link rel="alternate" type="text/markdown"> always points to a .md URL.
4. On servers where .md URLs are blocked (e.g. by nginx rules), the advertised link returns a 403 or 404, even though ?_format=markdown works fine.

Proposed resolution

Add a "Alternate link URL format" configuration option to the markdownify settings form (/admin/config/services/markdownify) that lets site administrators choose which URL format is used in the
meta tag:

- .md extension — e.g. /node/1.md (current behavior, remains the default)
- Query parameter — e.g. /node/1?_format=markdown
- Markdownify prefix — e.g. /markdownify/node/1

All three formats remain functional for accessing Markdown content regardless of this setting; it only controls which one is advertised in the HTML head.

Remaining tasks

- Review the patch.
- Decide whether additional URL formats should be supported (e.g. path alias variants via markdownify_path).
- Add test coverage for the new configuration option.

User interface changes

A new "Alternate link URL format" radio button group is added to the markdownify settings form at /admin/config/services/markdownify, with three options:
.md extension, Query parameter, and Markdownify prefix.

API changes

None.

Data model changes

A new link_format string key is added to the markdownify.settings config object. Accepted values: md_extension (default), query_parameter, markdownify_prefix.