Add datafusion website

[houqp]

Following the advice from @kszucs : https://lists.apache.org/thread.html/r5c9341fe5360ae249532724da2bf92bd2ed661d1c58f599212e82107%40%3Cdev.arrow.apache.org%3E.

This is the document build from https://github.com/apache/arrow-datafusion/tree/master/docs.

I will add link to the home page header as follow up PR after the first version of the page have been deployed and validated.

Here is what the page looks like from local build:

[kszucs]

LGTM, thanks @houqp!

[kszucs]

It is available at: https://arrow.apache.org/datafusion/

Is the website update process documented in the datafusion release guide?

What do you think about placing the datafusion docs page under the docs dir, so it would be reachable at: https://arrow.apache.org/docs/datafusion/ ?

[houqp]

Thanks @kszucs for being thorough and asked for a documentation update :) I was planning to add docs for the release step later this week after I validated the process with a successful release.

The reason I kept it under /datafusion path was due to the feedback from apache/arrow#10982 (comment). From a grouping point of view, it seems more logical to have all sub paths within the docs folder to be linked in https://arrow.apache.org/docs/. Perhaps a better path for us would be https://arrow.apache.org/datafusion/docs/?