updated images in mantid basic course

[yusufjimoh]

Description of work

• update outdated images in mantid basic course
• The color highlighting in https://docs.mantidproject.org/nightly/_images/PlotOptionsCurves.png does not seem to convey any additional meaning, so I chose to omit it in the new image
• https://docs.mantidproject.org/nightly/_images/PlotSettings.png has been updated in a different PR: updated images in mantid basic course #38802

Fixes #38050

To test:

[sf1919]

It looks like this is currently failing because there is a problem with the new image. It is unconnected with LoadMuonNexus3 issues.

09:02:50 /jenkins_workdir/workspace/pull_requests-doc-tests/docs/source/plotting/1DPlotsHelp.rst:76: WARNING: image file not readable: images/PlotOptionsCurves.png [image.not_readable]
09:02:50 /jenkins_workdir/workspace/pull_requests-doc-tests/docs/source/plotting/1DPlotsHelp.rst:169: WARNING: image file not readable: images/PlotOptionsCurves.png [image.not_readable]
09:02:50 /jenkins_workdir/workspace/pull_requests-doc-tests/docs/source/plotting/WaterfallPlotsHelp.rst:85: WARNING: image file not readable: images/PlotOptionsCurves.png [image.not_readable]

[yusufjimoh]

Thanks @sf1919