Combine tutorial and presentation pages into resources, add citation to references

[bettina-gier]

Added a Resource page which currently includes the tutorial and presentation pages and links to the community github currently. I envision that later on it will also include a pptx file about ESMValTool to include in presentations and potentially the logo for easy download.

Additionally after discussing with Lukas I added some more citation to the reference page alongside a bibtex file for all the papers.

Closes #81
Closes #21

Link to documentation:

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[lukruh]

I built and tested the changes locally. Looks good. Links and download of bib file is working. Well done.

[bettina-gier]

@axel-lauer can you do a quick check on this so we can get it merged, especially the sentence added for the reference? It's based on the License part of the documentation.

[axel-lauer]

Looks already very good. Here a few a small suggestions.

[bettina-gier]

Thanks for the review! With changes to the reference sentence, do we change the paragraph in the Documentation as well, or do we just link it to the website to not have to maintain it twice?

[axel-lauer]

Thanks for the review! With changes to the reference sentence, do we change the paragraph in the Documentation as well, or do we just link it to the website to not have to maintain it twice?

I think I would leave the documentation as is. I do not see a problem in the slight difference in the wording. I think once the conflict in _config.yml has been solved, this can be merged.

[bettina-gier]

done!

[axel-lauer]

Thanks @bettina-gier ! Looks all good now!