[DOC] update download button in tutorial and model zoo

[szha]

Description

doc website fixes.

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)

Changes

• update download button in tutorial and model zoo. download buttons are now after the first header in the form of a button.
• allow horizontal scroll when there is content overflow. (e.g. the huge table in language model)
• make navbar responsive.
• make header links ¶ appear when hovering the title.

Comments

• Require Theme fixes mli/mx-theme#3 to be merged first.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (download_button@83df8ff). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #746 into master will increase coverage by 0.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
+ Coverage   90.21%   90.63%   +0.41%     
==========================================
  Files          64       64              
  Lines        6233     6233              
==========================================
+ Hits         5623     5649      +26     
+ Misses        610      584      -26

Impacted Files	Coverage Δ
src/gluonnlp/data/stream.py	89.61% <0%> (+0.54%)	⬆️
src/gluonnlp/data/word_embedding_evaluation.py	96.96% <0%> (+0.75%)	⬆️
src/gluonnlp/data/batchify/batchify.py	96.29% <0%> (+1.23%)	⬆️
src/gluonnlp/data/dataset.py	98.41% <0%> (+1.58%)	⬆️
src/gluonnlp/data/transforms.py	80.28% <0%> (+3.01%)	⬆️
src/gluonnlp/data/corpora/wikitext.py	100% <0%> (+5.17%)	⬆️
src/gluonnlp/model/parameter.py	100% <0%> (+8%)	⬆️

[mli]

Job PR-746/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/1/index.html

[mli]

Job PR-746/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/2/index.html

[eric-haibin-lin]

The TOC sidebar looks fine here @szha

[mli]

Job PR-746/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/3/index.html

[mli]

Job PR-746/4 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/4/index.html

[mli]

Job PR-746/5 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/5/index.html

[mli]

Job PR-746/6 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/6/index.html

[mli]

Job PR-746/8 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/8/index.html

[mli]

Job PR-746/9 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/9/index.html

[mli]

Job PR-746/10 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/10/index.html

[mli]

Job PR-746/11 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/11/index.html

[mli]

Job PR-746/13 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-746/13/index.html