⚠️ Migrate needs_extra_links to needs_links

[chrisjsewell]

This PR introduces a new dict-based needs_links configuration and deprecates the list-based needs_extra_links.
The change follows the plan in #1646 and uses the same migration pattern as needs_extra_options → needs_fields (commit fc41bf2).

Motivation

• Consistent API design with needs_fields (dict-based config)
• Enables a description field for link types (output in schema/needs.json)
• Cleaner configuration syntax

Changes

New Configuration Format

# New (needs_links)
needs_links = {
    "blocks": {
        "description": "Blocking relationship between needs",
        "incoming": "is blocked by",
        "outgoing": "blocks",
    },
}

# Deprecated (needs_extra_links)
needs_extra_links = [
    {
        "option": "blocks",
        "incoming": "is blocked by",
        "outgoing": "blocks",
    },
]

Backward Compatibility

• needs_extra_links continues to work but emits a deprecation warning
• Internal processing converts both formats to the same representation
• Error messages reference the correct config option based on source

New Features

• description field: Link types can now include a description that appears in the JSON schema output
• Dict-based syntax: Link name is the key, making configuration cleaner

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.83%. Comparing base (4e10030) to head (32ff463).
⚠️ Report is 248 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/needs.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1649      +/-   ##
==========================================
+ Coverage   86.87%   88.83%   +1.95%     
==========================================
  Files          56       70      +14     
  Lines        6532     9952    +3420     
==========================================
+ Hits         5675     8841    +3166     
- Misses        857     1111     +254     

Flag	Coverage Δ
pytests	88.83% <95.83%> (+1.95%)	⬆️

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[ubmarco]

Just minor things