Updates to documentation

[jdebacker]

This PR makes the following updates to the Tax-Calculator documentation:

1. Removes the "Upgrading to Tax-Calculator 2.0" section of the Jupyter Book (per Issue Remove or update "Upgrading to Tax-Calculator 2.0" documentation page #2523).
2. Updates the documentation of iitax per Issue iitax documentation appears to need updating: Issue on page /guide/output_vars.html #2514.
3. Adds Google Colab launch to Jupyter Book, per Issue Add Colab and Binder launch buttons to JupyterBook documentation #2429.
4. Adds behresp to the environment.yml` file so that recipes can build from environment, per Issue Add behresp to taxcalc-dev environment #2426
5. Updates labels for tables per Issue TaxBrain Issue: Single term for returns/filers in tables #2396.
6. Drops use of disutils.core in the setup.py file per suggestion in Issue Drop distutils.core from setup.py #2428.
7. Update recipes to ensure they work on Google Colab and use reform files on the web (see Issue Add Colab and Binder launch buttons to JupyterBook documentation #2429).

[codecov]

Codecov Report

Merging #2592 (f03a19c) into master (7d96420) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2592   +/-   ##
=======================================
  Coverage   98.46%   98.46%           
=======================================
  Files          14       14           
  Lines        2611     2611           
=======================================
  Hits         2571     2571           
  Misses         40       40           

Flag	Coverage Δ
unittests	98.46% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
taxcalc/utils.py	97.79% <100.00%> (ø)

[jdebacker]

@MattHJensen I've taken off the "WIP" and marked this as ready for review. There are two questions outstanding to people who opened issues that are addressed here. But those are just confirmation - I believe I addressed their issues.

The biggest change to note here -- for Google Colab functionality, recipes now pull from reform json files via URL rather than relative file paths. That does assume a user of those notebooks is connected to the internet.

[MaxGhenis]

.

[MattHJensen]

Looks great, thanks for all of these cleanups and improvements, @jdebacker and thanks for review @MaxGhenis.