Provide links to previous Doxygen documentation

[ischoegl]

Changes proposed in this pull request

Make previous versions of Doxygen documentation easier to find (from current Doxygen page).

Also, fix a couple of Doxygen issues that prevent proper rendering of math equations or cross-references.

While there are - at least some - other projects that have implemented a version switcher for Doxygen documentation (example: OpenCV), this would presumably require regeneration of old documentation. Creating a page with links is a much simpler solution.

Addresses Cantera/enhancements#179

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

[speth]

Seems straightforward enough. I guess updating this needs to be added to the list of things done for each release, though at least in this case it's something that can be done after a release.

One question: do you want to add a link to the dev/latest docs?

[ischoegl]

One question: do you want to add a link to the dev/latest docs?

Great suggestion! (although it dismisses the review)