Tweak community content

[ischoegl]

As a minor follow-up to #269, this PR proposes the following:

• Move Cantera citation to a separate page to make it easier to locate.
• Move 'donation' section to the new 'getting involved' page.
• Minor reformatting.

[speth]

Thanks, @ischoegl. The old location for the citation information definitely felt "off" to me, and I think this will make it easier for people to find.