Commit Document and related objects every doc iteration

[HiromuHota]

Description of the problems or issues

Is your pull request related to a problem? Please describe.

See #494.

Does your pull request fix any issue.

Fix #494.

Description of the proposed changes

A clear and concise description of what you propose.

Test plan

A clear and concise description of how you test the new changes.

Checklist

• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] I have updated the CHANGELOG.rst accordingly.

[codecov-commenter]

Codecov Report

Merging #495 into master will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
- Coverage   85.85%   85.85%   -0.01%     
==========================================
  Files          88       88              
  Lines        4568     4567       -1     
  Branches      853      853              
==========================================
- Hits         3922     3921       -1     
  Misses        464      464              
  Partials      182      182              

Flag	Coverage Δ
#unittests	85.85% <100.00%> (-0.01%)	:arrow_down:

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

Impacted Files	Coverage Δ
src/fonduer/parser/parser.py	92.97% <ø> (-0.07%)	:arrow_down:
src/fonduer/utils/udf.py	88.78% <100.00%> (+0.21%)	:arrow_up: