Make `--doc_mode` image regression thresholds configurable

[banesullivan]

pytest-pyvista already lets regular image tests control comparison thresholds through VerifyImageCache(error_value=..., warning_value=...), but --doc_mode still uses hard-coded defaults of 500.0 and 200.0.

In practice, this makes downstream docs builds flaky. In PyVista's CI we are seeing doc image comparisons fail just over the line, for example:

sphx_glr_openfoam_cooling_001_vtksz Exceeded image regression error of 500.0 with an image error equal to: 513.5843137255087

see pyvista/trame-pyvista#62

Current behavior

doc_mode.py compares images using the module defaults:

pytest-pyvista/pytest_pyvista/pytest_pyvista.py

Lines 47 to 48 in 144eafb

	DEFAULT_ERROR_THRESHOLD: float = 500.0
	DEFAULT_WARNING_THRESHOLD: float = 200.0

And they are used directly in doc mode here:

pytest-pyvista/pytest_pyvista/doc_mode.py

Lines 553 to 554 in 144eafb

	allowed_error=DEFAULT_ERROR_THRESHOLD,
	allowed_warning=DEFAULT_WARNING_THRESHOLD,

As far as I can tell, there is no CLI or ini option for overriding these values in doc mode today and we cannot set the module level vars since doc_mode.py imports its own copies of those name

Proposed behavior

Expose doc-mode thresholds through config, ideally using the same config patterns the plugin already uses elsewhere.

Any of these would solve the problem:

1. Add common threshold options and allow doc_-prefixed overrides, for example error_threshold / warning_threshold plus doc_error_threshold / doc_warning_threshold.
2. Add doc-specific CLI flags such as --doc_error_threshold and --doc_warning_threshold.
3. Keep the current defaults, but let downstream projects opt into slightly higher thresholds when their docs images are known to be noisier in CI.

It looks like doc_mode.py could read the thresholds from config via _get_option_from_config_or_ini(...) instead of using the imported defaults directly. That would line up with the rest of the plugin's doc-mode configuration.