Allow docs preview to be generated even if docs build fails

[user27182]

Describe what maintenance you would like added.

Previewing the docs with @pyvista-bot preview is great, but a preview is currently only generated if the build succeeds. However, even if warnings or errors are generated by the build (and the build "fails"), the docs are usually still in a viewable state, but can't be viewed simply because the failure causes the bot not to generated the preview.

Further, with build times taking 2hrs recently (see #5868), it can be painful to wait 2hrs for a preview only for it not to be generated because of a simple rst formatting warning.

As a workaround, draft PRs can temporarily remove the -W --keep-going option from the makefile:

pyvista/doc/Makefile

Line 8 in a36f649

DEFAULTS = -w sphinx_warnings.txt -W --keep-going

in order to allow the build to succeed with warnings, thus allowing the bot to generate the preview. But, it would be nice not to have to use a workaround like this.

Links to source code.

No response

Pseudocode or Screenshots

No response