Fix doc links

[speth]

Changes proposed in this pull request

Fix some broken links that I discovered while working on the .htaccess files as part of Cantera/cantera-website#274

• Fix some broken links from Doxygen into the Sphinx docs
• Fix MATLAB Install guide points to broken link #1940

If applicable, fill in the issue number this pull request is fixing

Closes #1940

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

Thanks, @speth!