Fixes #220.

The docs build currently relies on a CI workaround that installs a TeX toolchain for Matplotlib-based example generation. The underlying issue is that the custom example gallery setup forces LaTeX rendering, and one example script also opts into usetex explicitly.

This patch removes that hard dependency by:

• switching the custom docs gallery builder to Matplotlib mathtext instead of usetex
• updating fig_volume_ratio.py to use setup_text_plots with usetex disabled
• adding a focused regression test for the docs-build path
• removing the extra TeX installation workaround from the docs CI job

Testing:

• python -m pytest -q -o addopts=astroML/doc/tests/test_issue220.py