docs: add how to create fdroid from default debian vagrant boxes

Merged username-removed-151489 requested to merge Bubu/fdroid-website:docs_create_basebox into master

The f-droid manual had no really good explanation on how to create a buildserver It just refers to the vagrant docs which don't have a very clear description either.

Also doing all those required steps manually is a time consuming and error prone process. Especially when it needs to be redone for new Debian releases.

So this commit documents an alternative way to prepare the relying on the debian prebuilt vagrant boxes. These are automatically build with packer and signed by the Debian Vagrant team:

We still need to do a few adjustments on the Debian vagrant boxes, namely install the virtualbox guest additions and provide a bigger hard disk. We also disable the default rsync behavior of the box as this conflicts with the fdroidserver cache directory mounting.

To do this we rely on two vagrant plugins and a small provisioning shell script which enlarges the root partition and root filesystem.

Motivation for this was of course

@eighthave What do you think about adding such an option directly to the makebuildserver script? So people can easily and automatically create an fdroid compatible even if the f-droid infrastructure is not available.