Set up new website built on MooseDocs system

[smpark7]

This PR addresses some tasks in issues #179 and #193 involving documentation and website setup. The changes include:

• more dropdown menus on the MooseDocs-based website for Getting Started and Documentation with links to Moltres and MOOSE documentation webpages;
• new home page for the Moltres website;
• updated installation instructions hosted on Getting Started/Install Moltres;
• automatically generated documentation webpage stubs (in markdown format) of all objects in Moltres (e.g. kernels, BCs, postprocessors, etc.) which are to be filled in another PR;
• modify the existing GitHub Action which previously deployed Doxygen documentation only. The new GH Action now builds MooseDocs html files for the new website and hosts the Doxygen documentation in /doxygen with a link to it from the navigation bar. The website will be hosted on https://arfc.github.io/moltres/ where our Doxygen documentation was previously hosted;
• update readme to link to the new website and remove duplicated information.

The task of building MooseDocs html files requires us to compile Moltres in the GitHub Action. This takes 30 minutes to run but I think it's better to exploit computational resources for reliability than having someone (aka me) build it locally and push to gh-pages every time there's a change to the docs.

Refer to https://smpark7.github.io/moltres/ to see how the website will actually look like.

For clarity, future tasks (already tracked in meta-issue #179) include:

• adding contributing and publications webpages;
• migrating existing content from http://arfc.github.io/software/moltres/wiki/;
• adding more tutorials;
• filling out the auto-generated documentation webpage stubs.

[smpark7]

@yardasol I added a "Contributing" page which includes instructions on how to contribute to Moltres and how to manually build the docpages. I also added a "Publications page that provides a list of publications which used Moltres.

I replaced the "Source documentation" page with a "Syntax" page which contains the same documentation links and also adds new links for each general class type (e.g. Kernels, BCs, etc.). The class type links lead directly to MOOSE-provided documentation which uses many of the large media files you found. I think this implementation is fair because MOOSE already provides good documentation for these kinds of MOOSE-wide features.

This PR is ready for another review. The preview website at https://smpark7.github.io/moltres/ will be ready once the gh-pages action finishes running on my fork https://github.com/smpark7/moltres/runs/6298699903?check_suite_focus=true.

[smpark7]

@munkm This is ready for your review. I incorporated your suggestions for reordering the list of publications by date and recommending git merge instead of git rebase. I hope to merge soon-ish because yet another person has had trouble with the outdated installation instructions #202. Thank you!

[munkm]

Thanks for the reminder @smpark7 ! I'm on it.

[munkm]

This looks great @smpark7 !!!! I have some minor comments and suggestions.

Thank you so much for redoing the website. I think the content is clear, readable, and accessible for users and developers alike.

[smpark7]

Thank you!