[Website] Improve website build workflow

[barry-jin]

Description

Improve workflow for building documentation website.

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [ ] All changes have test coverage
• [ ] Code is well-documented

Changes

• [ ] Feature1, tests, (and when applicable, API doc)
• [ ] Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

cc @dmlc/gluon-nlp-team

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-dev.s3-accelerate.amazonaws.com/PR1377/improve-docbuild/index.html

[codecov[bot]]

Codecov Report

Merging #1377 into master will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1377   +/-   ##
=======================================
  Coverage   71.05%   71.06%           
=======================================
  Files         107      107           
  Lines       12607    12607           
=======================================
+ Hits         8958     8959    +1     
+ Misses       3649     3648    -1     

Impacted Files	Coverage Δ
src/gluonnlp/data/tokenizers/yttm.py	82.75% <0.00%> (+0.86%)	:arrow_up:

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