[DOCS] We should have a islandora installation "landing page"

[ysuarez]

• If this is a request for new documentation, where in the existing documentation does this documentation belong?

Here are my draft suggestions for a final URL...
https://islandora.github.io/documentation/installation/ (this is not a valid URL yet)
or
https://islandora.github.io/documentation/installation/overview (this is not a valid URL yet)

• Describe your documentation request.

We could use a single page with an overview of all our ways to install Islandora 2, to make it easier to guide folks on what installation approach to take. Normally when I try to guide someone on installations I cannot send a single URL from our docs for installations, I have to send individual links for isle-dc, playbook, isle-site template.

BTW, @seth-shaw-asu suggested an initial breakdown of the installation in this overview page to be something like...

We have 3 types of installations 
* Manual
* Docker based
* Ansible based

Also, here is a spreadsheet that @rosiel created a spreadsheet to start breaking down the various installation types...

Islandora Install Structure Comparison
https://docs.google.com/spreadsheets/d/1tmvjyVPeQBM2LO7sq3lN0iMK0Cx_bIbhRp6k2glAxNs/edit#gid=0

• What user roles is this documentation intended for?

All user roles

• What implementations of islandora does this documentation request apply to?

Any related open or closed issues to this bug report?

This is a realted PR:
#2236