Remove qt-webengine dependency

[MialLewis]

Describe the outcome that is desired.

Remove qt-webengine as a dependency, as this package is not available on condo-forge.

Use QTextBrowser instead

QTestBrowser does not support JS, or CSS for that matter (uses QSS, which could be a complication): https://forum.qt.io/topic/105196/javascript-in-qtextedit/12

We currently use JS to render LaTeX images: https://github.com/mantidproject/mantid/blame/83a7b971adf2397bdbed2316e3431bcf913741bd/dev-docs/source/Standards/DocumentationGuideForDevs.rst#L29C4-L29C4
using MathJax: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.mathjax

This puts math as-is into the HTML files which then uses MathJax, a JS package, to load the LaTeX live. We can switch to rendering the LaTeX as images and injecting the images into the HTML (at the sacrifice of package size).

We could use typst as suggested by Shervin: https://github.com/typst/typst

Or may be simpler to stick with sphinx, as we already appear to support the use of sphinx.est.imgmath to remove the JS dependency: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.imgmath

if this approach does not work, we will fall back to using the users system browser.

[peterfpeterson]

One feature of the current help window is that it can render the pages from online. This allows for not installing the mantiddocs package which is currently ~150MB in size. If we move to something that cannot render css/javascript, we need to decide if either we require the offline docs, or if we should open a system web browser (e.g. xdg-open). The system browser option was available until qwebengine was put in place.

From this stack overflow post, it links to a fairly large amount of HTML and CSS that qtextedit supports.

Another question is if qtextedit can display mathml.

[MialLewis]

Building the packages locally, rendering as images (sphinx.ext.imgmath) increases package size by ~6.5% compared to using js (sphinx.ext.mathjax).

To me, it's not worth the effort of trying to retain js for this saving. I suggest we go ahead with removing javascript and moving to QTextBrowser, falling back to the local system browser if mantiddocs is not installed?