doc: Update information about vbguest plugin.

proj: Update ubuntu20.04 InvenioRDM VM.

This commit updates ubuntu20.04-inveniordm-development VM to up to date and makes it compatible with latest installation procedures.

proj: Update Debian Docker lab to Bookworm.

This commit updates debian-docker-lab to Bookworm, updates Docker
installation procedures to match to the latest pracitces, and updates
the README.md file to reflect the change.

doc: Update README.md file.

Since vagrant-vbguest is not maintained anymore, users are encouraged to
install VirtualBox guest additions themselves. This change is reflected
to document.

proj: Add buhtz’s BackInTime development machines.

- proj: Create a contrib file for contributed VMs.
- proj: Add buhtz’s BackInTime development VMs. Thanks!
- proj: Update .gitignore to ignore macOS specific files.
- docs: Update README.md.

fix: Correct folder naming of Sphinx builder.

debian11-sphinx-builder's folder was misnamed. This commit corrects this
error.

feat: Add debian12-maven machine to the repository.

This commit adds a new debian12-maven VM, which installs a vanilla Maven
on top of Debian 12. No other config is done.

proj: Debian Sphinx Builder machine has been added.

This is a machine with a single core and 1GB of RAM, and can build
Sphinx documentation sites out of the box.

Doc: Revise README.md to reflect new memory sizes.

README.md is improved to show how much RAM and VRAM each VM has.

Feat: Increase video memory of VMs to 128MB.

To be able to use greater desktops, video RAM of both VMs is bumped to
128MB.

Feat: Bump RAM of both Back in Time VMs to 4096MB.

Increasing RAM will allow a more smoother experience and allow more
realistic testing.

Fix: Add user creation script to Debian 11 VM.

This commit adds the user creation script to Debian 11VM. Also clarifies
the Post-Up messages of both Debian 11 and Debian 12 VMs.

Fluff: Change license filename to COPYING.

Fix: Improve machine up messages for readability.

Add Debian 11 & 12 Back in Time VMs.

This commit adds two new VMs for Back in Time testing. Both VMs run XFCE4
with latest Back in Time available in the repositories, so allows for turnkey
testing.

Debian 12 disabled automated VirtualBox Guest Additions installations since
the Vagrant VBGuest plugin is not compatible with Debian Testing yet.

Merge pull request #14 from hbayindir/debian-rust-playground

Upgrade debian-docker-lab to Debian/Bullseye64.

Upgrade debian-docker-lab to Debian/Bullseye64.

Merge pull request #13 from hbayindir/debian-rust-playground

Upgrade Rust Playground to use official Rust.

Upgrade Rust Playground to use official Rust.

Since the Rust packages coming from Debian are not the latest ones, and caused
problems during testing, a new VM using official Rust version is made.

This version works as it should for the cases I've tested.

Merge pull request #12 from hbayindir/debian-rust-playground

Add "Debian Rust Playground" VM.