doc: add reclaimspace documentation to index

Add the new reclaimspace.md documentation to the CSI-Addons section
of the main documentation index. This provides users with easy access
to information about space reclamation for filesystem volumes using
fstrim.

Assisted-by: AskBob <askbob@ibm.com>
Signed-off-by: Niels de Vos <ndevos@ibm.com>

Author: nixpanic