docs: document pruning feature for celestia-node

[jcstein]

for pruning is enabled by default, to run an archival node use:
--archival Enables archival mode, which disables pruning and enables the storage of all blocks.

[jcstein]

ok this is only for bridge and full nodes

celestia light start --p2p.network arabica --core.ip validator-1.celestia-arabica-11.com --core.port 9090 --archival
2025-09-25T16:17:31.042-0600	FATAL	module/pruner	pruner/flags.go:28	Archival mode is only supported for Full and Bridge nodes

[jcstein]

full nodes dont exist anymore