Fix Help Window path

[MohamedAlmaki]

Description of work

This PR fixes the path of the docs that caused an issue with the offline and online modes in the new Python help window. It also fixes a problem with Qt icons not being shown correctly on Windows. This issue doesn't resolve the conda installation bug on Windows.

Closes #39506.

To test:

• Development Build
  1- Build this branch locally
  2- Go to Mantid Help
  3- You will see the help docs in online mode and a warning that the docs were not found in the local path
  4- Build docs-html
  5- Rerun Mantid; it should now show the offline docs
  6- Check that all icons are shown correctly

• Release Build
  Check the unstable build is working as expected https://builds.mantidproject.org/job/build_packages_from_branch/1299

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Reviewer

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Gatekeeper

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[thomashampson]

I tried installing the Windows .exe from the linked build and it displays only the online docs, with this warning message:
Local docs path '../share/doc/html' from ConfigService ('docs.html.root') is invalid or not found. Falling back to Online Mode.

[MohamedAlmaki]

Thanks I will check

[MohamedAlmaki]

There was minor issue with the path hopefully this build will fix it: https://builds.mantidproject.org/job/build_packages_from_branch/1301/