Retire isle-dc in favour of Isle Site Template about documentation

[rosiel]

This was discussed at the Tech Call and TAG today, and while there were some concerns, it seems prudent to move forward in this direction.

It should be mentioned that the philosophy of the Site Template is starkly different from that of ISLE-DC. The Site Template is a download-it-once-and-its-yours repository, where updates are NOT available for download. On the other hand, ISLE-DC tries to keep you "up to date" in many ways (more than it probably should).

I think the sentiment at TAG was that we want to spend less time maintaining ways to start up islandora and more time on islandora code itself. The Site Template has less of a maintenance burden, so seems to be the better option. also it's less in the way of users learning Docker (as it doesn't hide any docker commands behind abstractions) and that's useful for getting people up to speed as they need to be to run islandora in production.

It was noted that the makefile contains many useful commands that are hard to execute otherwise (such as exporting your database or importing/exporting configs) and so having a makefile available - either part of the Site Template or elsewhere - with the relevant code would be appreciated.

It would be good to see a fleshed-out list (scoping doc, I guess) that lists the elements part of 4a and 5a/b of the above list.