src/sage_docbuild/conf.py: use absolute path to mathjax

Previously we copied mathjax to sphinx's _static output directory and
then referenced it with a path relative to _static. This commit
references it by absolute path on the filesystem, solving two
problems:

  1. It doesn't waste space; there are multiple _static directories,
     and mathjax was copied to all of them
  2. It avoids polluting _static with mathjax files

Closes: #30296

Author: orlitzky

src/sage_docbuild/conf.py

@@ -374,8 +374,7 @@ def set_intersphinx_mappings(app, config):
 if os.environ.get('SAGE_USE_CDNS', 'no') == 'yes':
     mathjax_path = "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js"
 else:
-    mathjax_path = 'mathjax/tex-chtml.js'
-    html_common_static_path += [MATHJAX_DIR]
+    mathjax_path = os.path.join(MATHJAX_DIR, 'tex-chtml.js')
 
 # A list of glob-style patterns that should be excluded when looking for source
 # files. They are matched against the source file names relative to the