Update documentation with reference to manual

[dfalster]

Recently I've been integrating various vignettes from austraits.build and traits.build into a manual, available at https://traitecoevo.github.io/traits.build-book/.

The The AusTraits Plant Dictionary (APD) is now also available at http://w3id.org/APD/

As a result, we should update the documentation in this package to point to these sources for

• structure of austraits
• dictionary of traits
• on readme, point to these resources in https://traitecoevo.github.io/austraits/index.html#show-us-some-support-

Can you handle this @fontikar ?