Add initial offline build instructions

[jschwe]

At the time of writing

• We don't have a release which provides the artifact described here yet. (the script to prepare the artifact is merged in tree, but still missing wiring-up in the release.yml workflow). Merging of this PR should probably be synced with the first release that does provide the tar-ball.

Testing:

# in the servo repository create a new clean worktree based on latest main
git worktree add ../servo-vendor-testing
cd ../servo-vendor-testing
# Create a vendored tar-ball (which would be provided by a servo release)
./etc/vendor_servo.py
mkdir ../offline-servo
cd ../offline-servo
# Extracts the tarball into the current directory
tar -xf ../servo-vendor-testing/servo.tar.gz
uv sync
# Now disconnect the internet and build (assuming bootstrap was run before on the machine)
./mach build