Add missing link to C++ documentation for Cantera 3.1.

Also, reformat release information as a table and add information about access to Cantera sources.

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

Relates to Cantera/cantera-website#267 PS: overlooked a link.

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