[MXNET-341] Added General Info to API Docs for Python and Gluon

[thomelane]

Description

With the aim to removing the main navigation link to Gluon, these changes are to provide extra context around Module vs Gluon and rehome the information.

Gluon link in main navigation is out of place, and only provides links to a basic about page and an external website (which is unintuitive from a user perspective). Suitable location would be API docs, and this can then be linked to from other pages such as FAQ.

Changes:

1. Moved content from Gluon/About (http://mxnet.incubator.apache.org/gluon/index.html) to API/Python/Gluon (http://mxnet.incubator.apache.org/api/python/gluon/gluon.html)
2. Added Module vs Gluon description to API/Python (http://mxnet.incubator.apache.org/api/python/index.html)

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

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

[thomelane]

JIRA: https://issues.apache.org/jira/browse/MXNET-341

[thomelane]

@aaronmarkham what's the latest with changes of gluon homepage? good to merge this?

[aaronmarkham]

GTG! Ship it!

[thomelane]

@aaronmarkham @indhub can either of you merge this pr?

[aaronmarkham]

I can't... maybe @szha or @eric-haibin-lin can.