Fixed path bug with links in SVG inheritance diagrams

[ayshih]

Subject: Fixes links in SVG inheritance diagrams when those diagrams are in documents not at the base level

Feature or Bugfix

• Bugfix

Purpose

Currently, links in SVG inheritance diagrams only work when the documents are at the base level. That's because the URIs supplied to the diagram are relative to the document (as is appropriate for PNG output), but the SVGs live in the /_images folder, so the URIs need to be relative to that location. There is a hard-coded '../' that gets prepended to the URIs, which fixes only base-level documents. Documents in sub-directories result in SVG links that have one or more extraneous '../' that break the links.

Detail

This PR fixes the URIs to be relative to the image directory prior to making the SVG inheritance diagrams.

Relates

Fixes #10570

[ayshih]

Note that this PR fixes a bug in Sphinx that is unrelated to any use of third-party extensions (e.g., sphinx_automodapi).

Non-exhaustive list of issues that this PR closes: #3176, sunpy/sunpy#2097, astropy/astropy#4935, probabilistic-numerics/probnum#459

Related issues:

• Revert using SVG inheritance diagrams matplotlib/matplotlib#17913: matplotlib is choosing to use PNG inheritance diagrams because the links in SVG inheritance diagrams are broken
• Fix link target path for SVG inheritance diagrams #2484: Astropy's inheritance diagrams are all in subdirectories one level down, so that's why removing the prepended '../' happened to fix all of their inheritance diagrams. It was missed at the time that there was in fact a bug in Sphinx itself, presumably because only base-level inheritance diagrams had been tested.
• Broken links in class inheritance diagram docs. astropy/photutils#345: The significance of why moving the inheritance diagram from a subdirectory to the base level fixed the SVG links was not recognized at the time.

[ayshih]

Closing this PR since #10614 is more encompassing