[Doc] Update the download page with 1.5.1 release

[TaoLv]

Description

(Brief description on what this PR is about)

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$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

[TaoLv]

@aaronmarkham @ThomasDelteil @szha Please help to review and merge. Thanks!

[aaronmarkham]

Does the guide mention when the links transition to archive? I believe that's part of the release maintenance too. Otherwise, this looks fine.

[ThomasDelteil]

@TaoLv thanks for taking care of the release.
You also need to update this line: https://github.com/apache/incubator-mxnet/blob/master/docs/static_site/src/_includes/get_started/get_started.html#L27

and this table:
https://github.com/apache/incubator-mxnet/blob/master/docs/static_site/src/_includes/get_started/pip_snippet.md#L6

@aaronmarkham how is this table created? Do we have a master somewhere?

[TaoLv]

@ThomasDelteil, I have changed the versions and the pip package table. Could you please take a look again?
@aaronmarkham, Not sure I understand the question. The artifacts of these download links have already been archived here: https://apache.org/dist/incubator/mxnet/1.5.1/.

[TaoLv]

Is it good to merge now? @ThomasDelteil @aaronmarkham