Improve how help files look

[ilectra]

• Investigate how to make monospaced text look nicer in the matlab html helpfiles. See discussion starting in #204.
• Some images also don't look that nice in the documentation. They're too large for the browser window. For examples, look at the acoustic solver tutorials. A good starting point would be the various options in publish, as it's used in GenerateDocumentation.m to create the html files.
• In addition to the above, the following image in t02_MoreAcousticSolverExamples1D looks especially bad in the markdown file.