docs: Fix furo requirement after recently breaking docs (AcademySoftwareFoundation#4743)

PR AcademySoftwareFoundation#4736 bumped the version of sphinx we use on readthedocs, and that
was incompatible with the old version of furo we specified.

I'm not sure how the problem was not caught by our "docs" workflow.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/doc/requirements.txt

@@ -1,4 +1,4 @@
 sphinx >= 8.0
 breathe == 4.36.0
 sphinx-tabs
-furo==2022.6.21
+furo==2024.8.6