Sharding: Advice about increasing/decreasing shards

On improving CrateDB guidelines about sharding, @matriv suggested another improvement. Thanks.

> Additionally I believe we could mention the increase/decrease number of shards after table creation [^1], maybe with a special mention to [^2], since if this value is not set correctly it might prevent users from correctly increasing their shards.

[^1]: https://cratedb.com/docs/crate/reference/en/latest/general/ddl/alter-table.html
[^2]: https://cratedb.com/docs/crate/reference/en/latest/sql/statements/create-table.html#sql-create-table-number-of-routing-shards

_Originally posted by @matriv in https://github.com/crate/cratedb-guide/pull/355#pullrequestreview-3309148469_