Documentation Overhaul

[hayesall]

This PR reorganizes the sphinx documentation, abbreviated as follows:

• Home page is leaned down substantially: now it only includes a brief motivation, a gif of the code in action, and links to the bugtracker and issues page. The bottom of the page links to the more in-depth guides.
• Table of contents (on the right of each page), now links to the getting started information and the module-level documentation.
• each module (corpus, parse, textprocessing) has its own page now.
• New "Getting Started" section, including a brief overview of setting up a development environment, installing the software, discussing the relations encoded by the software, and an updated learning task.

Since one of the example files is the U.S. Declaration of Independence, the learning task now reflects the task of determining if a sentence is part of the "List of Grievances" against King George III.

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This PR modifies the example_files/doi.txt in order to change some of the end-of-line colon characters to periods. This change was made since the default nltk sentence-tokenizer does not consider colons to be end-of-sentence characters.

[hayesall]

@skinn009 I'd really appreciate a close review on this one, particularly in terms of the written content and whether the tutorial makes sense.

Pending acceptance, squashing these would probably be best.

[codecov-io]

Codecov Report

Merging #19 into development will not change coverage. The diff coverage is n/a.

@@             Coverage Diff              @@
##           development      #19   +/-   ##
============================================
  Coverage        95.48%   95.48%           
============================================
  Files                8        8           
  Lines              310      310           
============================================
  Hits               296      296           
  Misses              14       14

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