[FEATURE]Add new command for building website locally

[chenw23]

Description

Currently, the L62 to L81 in the old Makefile is preventing users from building the website locally. It gives the following errors:

for f in docs/examples/word_embedding/word_embedding_training.md docs/examples/word_embedding/word_embedding.md docs/examples/machine_translation/transformer.md docs/examples/machine_translation/gnmt.md docs/examples/sentence_embedding/elmo_sentence_representation.md docs/examples/sentence_embedding/bert.md docs/examples/language_model/train_language_model.md docs/examples/language_model/use_pretrained_lm.md docs/examples/sentiment_analysis/sentiment_analysis.md docs/examples/sentiment_analysis/self_attentive_sentence_embedding.md docs/examples/sequence_sampling/sequence_sampling.md ; do \
                FILE=`echo $f | sed 's/docs\///g'` ; \
                DIR=`dirname $FILE` ; \
                BASENAME=`basename $FILE` ; \
                HTML_BASENAME=`echo $BASENAME | sed 's/md/html/'` ; \
                IPYNB_BASENAME=`echo $BASENAME | sed 's/md/ipynb/'` ; \
                TARGET_HTML="docs/_build/html/$DIR/$HTML_BASENAME" ; \
                echo "processing" $BASENAME ; \
                sed -i "s/$IPYNB_BASENAME/$BASENAME/g" $TARGET_HTML; \
        done;
processing word_embedding_training.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing word_embedding.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing transformer.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing gnmt.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing elmo_sentence_representation.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing bert.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing train_language_model.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing use_pretrained_lm.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing sentiment_analysis.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing self_attentive_sentence_embedding.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
processing sequence_sampling.md
sed: 1: "docs/_build/html/exampl ...": extra characters at the end of d command
make: *** [docs] Error 1

These errors are unfriendly to users who want to build the website locally. So I invented a new command to circumvent such codes with the most concise way.

It doesn't affect all the existing command and CI.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• 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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (patch-4@e1846f8). Click here to learn what that means.
The diff coverage is n/a.

[mli]

Job PR-1198/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1198/1/index.html