Set up MooseDocs files for documentation

[smpark7]

Partially fulfills #179

This pull request adds MooseDocs configuration files which generate a simple static site for Moltres. The site holds documentation for Moltres source files not available in the existing Doxygen documentation in https://arfc.github.io/moltres. To build the site in this current state, users have to run the following commands within the MOOSE Conda environment:

cd doc/
./moosedocs.py generate MoltresApp
./moosedocs.py build --serve

and navigate to http://127.0.0.1:8000/ on their browser.

The remaining tasks towards building a fully fledged website for Moltres are:

1. Filling up the "Getting Started", "Help", and other pages as required
   • This may involve migrating existing Moltres webpages in arfc.github.io into this website
2. Deploying the website to be accessible online
3. Adding a link to our existing Doxygen-built website, or incorporating the Doxygen webpages directly into the website
4. Filling up the documentation "stubs". Running the commands above automatically generates these "stubs" that contain information on the input parameters of each class. We should strive to include more information on the respective physics that each class models and examples of how they're to be used.

I'll add these sub-objectives to #179.

[pep8speaks]

Hello @smpark7! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/moosedocs.py:

Line 2:1: E265 block comment should start with '# '
Line 3:1: E265 block comment should start with '# '
Line 4:1: E265 block comment should start with '# '
Line 5:1: E265 block comment should start with '# '
Line 6:1: E265 block comment should start with '# '
Line 7:1: E265 block comment should start with '# '
Line 8:1: E265 block comment should start with '# '
Line 9:1: E265 block comment should start with '# '
Line 15:80: E501 line too long (107 > 79 characters)
Line 17:80: E501 line too long (93 > 79 characters)
Line 19:80: E501 line too long (90 > 79 characters)
Line 27:1: E402 module level import not at top of file

[smpark7]

Given that I copy-pasted moosedocs.py directly from MOOSE, we can ignore pep8speaks.

[gwenchee]

LGTM!