[website] global search feature needed

[aaronmarkham]

Description

The website would benefit from a global search feature.

The current search feature provided by sphinx is only for the Python API, is nested deep in the site, and also happens to come with some legacy issues from its origins:
https://github.com/mli/new-docs/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+search

It might be better to just swap out with a more robust feature that is based on an outside indexing of the site and all of its microsites.

[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended label(s): Feature, Doc

[ChaiBapchya]

@ys2843 This is done!!!

[ys2843]

Thanks, PR: #18288. We can close this issue now