Add 'Scripted Upgrades'-page

[OleVik]

A guide to make upgrading multiple Grav-installations easier, using Deployer.

This PR adds a page under the 'CLI Console'-pages of the Docs named 'Scripted Upgrades', which is a tutorial on how to "automate" upgrades of multiple, separate installations of Grav. "Automate" in the sense that a single command can upgrade numerous independent sites. Whilst it is fairly comprehensive in detail, the language is colloquial enough to (hopefully) be understood by novice users.

It is based upon a discussion and walk through with @paulhibbitts on Gitter, as well as the brief example on the forums (now outdated, as the included example is improved). The script is aimed at being run on a shared server, but outlines that it can be extended to be ran locally or cross-server as well.

It has been tested on a few different configurations of shared server, running the typical LAMP-stack, as well as with a local Windows-installation. As a task-runner it is very simple in setup, requiring only 30 lines at minimum and Deployer on the back-end to run.

Input on changes or clarifications are very welcome.

[OleVik]

And if anyone knows how to squash those damn Merge-commits in GitHub Desktop, I'd be very happy to get rid of them.

[flaviocopes]

Thanks, that's cool! I didn't know about Deployer.

I merged the PR using squash&merge, since GitHub allows such functionality, it generates a single commit from any PR (a20de52).

You can also squash the commits yourself (e.g. http://eli.thegreenplace.net/2014/02/19/squashing-github-pull-requests-into-a-single-commit), especially useful when merging feature branches without making a github PR.