Support sub-attribute selection for structured attributes in publish configuration

[opt12]

Summary

The publish list in .doorstop.yml currently only supports simple attribute names (strings). When a published attribute contains a list of dictionaries (structured attribute), Doorstop dumps the raw Python object representation, which renders incorrectly in HTML output.

Proposed Enhancement

Extend the publish entry format to allow selecting specific sub-attributes of structured attributes, including an option to render them as hyperlinks.

Example of a structured extended attribute:

# Item file (e.g. SRS-001.yml)
spec-refs-from:
  - file: specs/login.md
    section: "3.1 Login Process"
    anchor: 31-login-process
    url: https://gitlab.com/group/project/-/blob/main/specs/login.md#31-login-process

Without this feature, publishing spec-refs-from renders as a raw Python object dump. With this feature, specific sub-attributes can be selected for clean output.

New syntax:

attributes:
  publish:
    - simple-attribute          # existing behavior – unchanged
    - attr: spec-refs-from      # structured attribute
      fields:
        - url: section          # {url_key: label_key} → rendered as hyperlink

Behavior

fields entry	Result
{url_key: label_key}	<a href="{url}">{label}</a>
fieldname	plain text value

Multiple list entries are separated by <br> in the output.

Backward Compatibility

Fully backward compatible. All existing string entries in publish behave exactly as before.

[jacebrowning]

attr feels a bit too jargony for a user-facing configuration file. What do you think about either of these?

attributes:
  publish:
    - simple-attribute
    - name: spec-refs-from
      fields:
        - url: section

or just mapping the attribute directly:

attributes:
  publish:
    - simple-attribute
    - spec-refs-from:
        fields:
          - url: section

[opt12]

Good proposal – I went with option 2.

The attribute name is now used directly as the key, which feels more natural and consistent with YAML conventions:

attributes:
  publish:
    - simple-attribute
    - spec-refs-from:
        fields:
          - url: section

All existing string entries remain fully backward compatible. The implementation and documentation have been updated accordingly.

[opt12]

integrated with PR #830