Quick start guide

[hepengfe]

This PR fixes #729.

Description of changes

• Merge overview.rst into README.md
• change markdown hyperlinks to the web link directly
• add file linking to README.md

Possible influences of this PR.

Describe what are the possible side-effects of the code change.

Test Conducted

Describe what test cases are included for the PR.

[codecov]

Codecov Report

Merging #730 (717bd27) into master (3c1425a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #730   +/-   ##
=======================================
  Coverage   81.18%   81.18%           
=======================================
  Files         248      248           
  Lines       18509    18509           
=======================================
  Hits        15026    15026           
  Misses       3483     3483           

Impacted Files	Coverage Δ
forte/data/data_pack.py	87.16% <ø> (ø)
forte/data/data_pack_dataset.py	95.83% <ø> (ø)

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[hunterhector]

Please pay more attention when you are writing tutorials. Why are you even using this Word class from a test case? It makes no sense.