Add dashboard user guide

[kazuhitoyokoi]

Hitachi colleagues have started to write a dashboard user guide because it is a little difficult for beginners to use the Node-RED dashboard. To share our progress with Node-RED community, I created this draft pull request. Comments from the community are always welcome.

[kazuhitoyokoi]

Currently, there is a table of contents only but we are going to publish the first commit (Introduction to dashboard) at the end of August.

[HiroyasuNishiyama]

Just some tidy up comments.

Thank you for your review. I have corrected the document according to your suggestions.