Help: Restore sphinx pre-5.0 compatibility

Revise commit 97d13c3053 (Help: Improve diagnostics documentation,
2026-04-30, v4.4.0-rc1~227^2~2) to switch `cmake.py`'s sphinx api usage
from 7.4+ features to 4.0+ features.

Replace `parse_text_to_nodes` and `parse_content_to_nodes` with
equivalent, backwards compatible sphinx code using existing api calls.

Signed-off-by: John Parent <john.parent@kitware.com>