[CI][DOC] Improve website build process + Fix doc

[barry-jin]

Description

1. Notebook fails compilation will be shown in CI and developers can download build artifacts to find the error message.
2. After #1356 is merged, tutorials question_answering and text_prediction_part2 will fail the notebook compilation because these notebooks need to be updated. Now both of them are updated.

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)
• [x] All changes have test coverage
• [x] 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

[codecov[bot]]

Codecov Report

Merging #1418 into master will decrease coverage by 0.18%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1418      +/-   ##
==========================================
- Coverage   85.24%   85.05%   -0.19%     
==========================================
  Files          53       53              
  Lines        6954     6987      +33     
==========================================
+ Hits         5928     5943      +15     
- Misses       1026     1044      +18     

Impacted Files	Coverage Δ
src/gluonnlp/attention_cell.py	80.39% <0.00%> (ø)
src/gluonnlp/data/loading.py	78.11% <0.00%> (-5.29%)	:arrow_down:
src/gluonnlp/utils/lazy_imports.py	66.08% <0.00%> (+6.33%)	:arrow_up:
src/gluonnlp/base.py	96.66% <0.00%> (+6.66%)	:arrow_up:

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[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1418/improve-ci2/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1418/improve-ci2/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1418/improve-ci2/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1418/improve-ci2/index.html