Add widgets documentation to tardis docs

[jaladh-singhal]

It adds documentation of widgets to tardis docs by creating a directory tardis_widgets having:

• using_widgets.rst - For description about how to use widgets and their features
• generating_widgets.ipynb - Demo notebook to generate widgets

Note: This PR is branched on PR#1264, so should only be merged after #1264 gets merged

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Codecov Report

Merging #1280 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1280      +/-   ##
==========================================
- Coverage   81.75%   81.57%   -0.18%     
==========================================
  Files          44       46       +2     
  Lines        3699     3799     +100     
==========================================
+ Hits         3024     3099      +75     
- Misses        675      700      +25     

Impacted Files	Coverage Δ
tardis/plasma/properties/ion_population.py
tardis/io/atom_data/atom_web_download.py
tardis/montecarlo/formal_integral.py
tardis/model/density.py
tardis/model/base.py
tardis/io/config_internal.py
tardis/io/parsers/stella.py
tardis/widgets/util.py
tardis/io/atom_data/base.py
tardis/io/util.py
... and 80 more

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[jamesgillanders]

great work @jaladh-singhal! Very well documented. Happy for this to be merged, pending review of my comments

[MarkMageeAstro]

One minor comment regarding grammar, but otherwise looks good.

[MarkMageeAstro]

Looks all good to me.