First of all, learn to utilize volume groups.
Not matter of the backed devices (md devices, disk devices, fc luns etc), the flexibility offered is substantial.
As for your situation, you can replace all disks in array with bigger ones then grow.
Growing - Linux Raid Wiki
This will require work and has its risks, additional inspection should be done when dealing with root and boot filesystems (/)
For learning, pop a virtual machine, make similar setup and test the scenarios.
What i recommend is examining what is taking so much space on
/dev/md2 21G 20G 211M 99% /
Clean up the space and/or make symlinks to /home from stuff which you pile in root filesystem, and it is not related to system, but your actions (torrents, downloads etc.)
Again, having volume group instead of /home over md device would have some advantages as well.
I would like to point out that having root on 99% is not advisable or prudent to have
Hope that helps
Regards
Peasant.