Updates for Docusaurus 2

[vtlim]

Changes in this PR

• Removed _data (content now in static/js)
• Updated Python publishing script (see details below)
• Updated link in druid-powered.md since the file moved
• Updated CSS for styling improvements

Script update details

Makes the source directory a command line argument, shows the full path of the source directory during verification step, and some cleanup.

For an invalid source, the output looks like this

$ python do_all_things.py -v 26.0.0

Checking docs source from the following:
Repo:	'/Users/victorialim/Documents/work_repos/druid'
Error: Supply a valid path for apache/druid in the '--source' flag

For a valid source, the output looks like this

$ python do_all_things.py -v 26.0.0 --source ../../druid-apache 

Checking docs source from the following:
Repo:	'/Users/victorialim/Documents/work_repos/druid-apache'
Branch:	'docusaurus2'

Is the listed docs source correct? (y/n) y
Is 26.0.0 going to be the highest version available for download? If yes, the docs will also be used for 'latest'. (y/n) y
Also copying the docs to docs/latest.
Installing Docusaurus 2...

[vogievetsky]

Thank you for the changes!

[317brian]

@vtlim we forgot to update the README for this change