Add reference to MESSAGEix manuscript on overview pages

[danielhuppmann]

This PR adds references to the MESSAGEix manuscript (Huppmann et al., 2019) on the landing and overview page, and adds the item to the various bibliography files.

[danielhuppmann]

@khaeru @volker-krey, I did not add any hyperlinks to the online MESSAGEix (framework) documentation, because I do not know what URL that page will have after the release (the current address https://message.iiasa.ac.at via RTD will be used for the overall documentation of MESSAGE at ENE, I guess?).

[khaeru]

@khaeru @volker-krey, I did not add any hyperlinks to the online MESSAGEix (framework) documentation, because I do not know what URL that page will have after the release (the current address https://message.iiasa.ac.at via RTD will be used for the overall documentation of MESSAGE at ENE, I guess?).

Since both are using Sphinx, we can take advantage of the built-in intersphinx extension:

A link like :ref:comparison manual <python:comparisons> will then link to the label “comparisons” in the doc set “python”, if it exists.

Someone will need to set this up and adjust such cross-links; unfortunately I don't have time to do so this week. Probably can be made a separate issue and addressed in the overall update PR or later.

[behnam-zakeri]

Thanks @danielhuppmann for the additions. Just a reminder that the reference to MESSAGEix paper was also updated in other PRs, i.e., #21 and #29. So, the final merge of all these clean-up PRs should account for this and remove redundant references.