docs/Run your node: Add dedicated section about pruning

[martintomazic]

We should add a dedicated section about pruning, probably under Maintenance section?

1. How to configure it for consensus and paratime nodes.
   • We have this currently under hardware requirements.
2. Suggested values for the pruning.
3. How to guarantee optimal disk usage:
   • For now by pruning from the start.
   • Later we will add the oasis-node storage compact command there, possibly "offline pruning".
4. Explain that with many versions, pruning is quite slow (especially for the runtime state) and that operators should consider (temporary) increasing BadgerDB block cache size.
   • Especially when keeping months of the data, if possible. Pending some more experiments from Pruning of runtime state is too slow oasis-core#6334.
5. How much extra free disk space operator should have available to guarantee compaction success.

[martintomazic]

Assigning @amela as requested on the slack. Note some things are still WIP.

[matevz]

Please also update https://github.com/oasisprotocol/oasis-core/blob/master/docs/oasis-node/cli.md