[DOC] update installation guide with selectors

[szha]

Description

update installation guide by adopting the selectors from CV

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

• update installation guide with selectors

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #966 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #966   +/-   ##
=======================================
  Coverage   89.87%   89.87%           
=======================================
  Files          67       67           
  Lines        6371     6371           
=======================================
  Hits         5726     5726           
  Misses        645      645

[mli]

Job PR-966/6 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-966/6/index.html

[leezu]

Thanks @szha

How about left-aligning the buttons?

The claim that Python 2 is supported is wrong.

[mli]

Job PR-966/7 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-966/7/index.html

[mli]

Job PR-966/9 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-966/9/index.html

[mli]

Job PR-966/10 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-966/10/index.html