Improve Task documentation #1003

Just adding this as a todo to make sure it doesn't get lost.

We discussed and agreed that it would be good to flesh out the documentation for tasks. @renkun-ken highlighted the following:

Yes, of course. I'll add an entry to Features which points to Tasks in wiki. If you are interested, you could directly edit the wiki page.

I'm thinking that we need to cover the following:

• Very very basic introduction to what tasks are and how to launch them (then point to vscode docs for more details)
• Highlight the pre-build R tasks (test, install, document, etc) and point out how they can be accessed via "run test tasks" (also highlight the prerequisite for a DESCRIPTION file
• Also highlight that the Test task integrates with the problems pane to highlight the exact unit tests that failed
• Show how users can customise the default tasks using the spanner icon and then how they can just thinks such as the R options i.e. --silent
• Finally show users how our exported task definition allows them to right custom tasks purely in R code

Maybe this is a bit much but it would cover everything. Any other thoughts ?