🐛 FIX needflow graphviz links

[chrisjsewell]

Links are now initially calculated relative to the containing document.

For PNGs, the links are defined as https://developer.mozilla.org/en-US/docs/Web/HTML/Element/map in the document, so this is correct.

For SVGs, the graphs are extracted to external files, and in this case the links are modified by sphinx to be relative to the SVG file
(from sphinx 7.2 onwards, see: sphinx-doc/sphinx#11078)

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 87.98%. Comparing base (4e10030) to head (25af27c).
Report is 81 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/directives/needflow/_graphviz.py	68.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1339      +/-   ##
==========================================
+ Coverage   86.87%   87.98%   +1.10%     
==========================================
  Files          56       60       +4     
  Lines        6532     7100     +568     
==========================================
+ Hits         5675     6247     +572     
+ Misses        857      853       -4     

Flag	Coverage Δ
pytests	87.98% <68.75%> (+1.10%)	⬆️

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