[EZ] Add doc sync to tox testenv:doc

[vincentschen]

Description of proposed changes

• Move the tox update command to the doc environment in tox, so tox -e doc will update docs, rather than `tox -e fix.

Test plan

• Manually updated docs/packages.json, ran tox -e doc, and saw updates to appropriate docs/*/*.rst

Checklist

Need help on these? Just ask!

• [x] I have read the CONTRIBUTING document.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] I have run tox -e complex and/or tox -e spark if appropriate.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Merging #1536 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1536   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files          55       55           
  Lines        2052     2052           
  Branches      338      338           
=======================================
  Hits         2001     2001           
  Misses         23       23           
  Partials       28       28