Move license file

[jdebacker]

This PR moves the license file out of the Jupyter Book documentation files to the root directory of the repo. Resolves Issue #2639.

[codecov]

Codecov Report

Merging #2706 (97664bc) into master (6798fba) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2706   +/-   ##
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  Coverage   98.54%   98.54%           
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  Files          14       14           
  Lines        2609     2609           
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  Hits         2571     2571           
  Misses         38       38           

Flag	Coverage Δ
unittests	98.54% <ø> (ø)

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Files	Coverage Δ
taxcalc/utils.py	97.79% <ø> (ø)

[jdebacker]

@MattHJensen This PR is ready for your review.

The change is to move license.md out of the Jupyter Book documentation and to the root directory so that it shows up on the main page of the GH repo.

If we still feel like it should be in those docs (e.g., at https://taxcalc.pslmodels.org) then I am happy to duplicate the file and have it in both places. But Jupyter Book can't handle compiling with files above the root directory for the book.

[MattHJensen]

@jdebacker, I think duplicate the license and then good to merge. Nice to have both places. License doesn't change, so duplication isn't a maintenance issue.

[jdebacker]

Per @MattHJensen's suggestion, the license file has been duplicated so it ships with the documentation as well.

[jdebacker]

@MattHJensen Do this PR look good now?