URL & documentation updates for 3.1 release

[speth]

Changes proposed in this pull request

• Update URLs to correspond to the new website organization coming with the Cantera 3.1 release. This PR handles those URLs that can be updated now, for links that are now internal within the version-specific documentation or are to other fully qualified URLs. A separate PR will handle those updates that can only occur simultaneous with the deployment of the new website and the 3.1 release.
• Add a documentation page covering the steps needed to make a new Cantera release (adapted/updated from the cantera-release-guide repo)
• Fix some typos in the documentation

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

Partially resolves #1748.

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. I can't comment on the release instructions, but the rest looks good to me!

[bryanwweber]

Thanks Ray!