[Doc][3.1] Add a link to the V2 Checkpoint specification in the DROP TABLE Feature doc

Backport of #2572. Support for dropping V2 Checkpoints was added in Delta 3.1. This PR makes sure that the V2 Checkpoint spec is linked in the doc about DROP Table Feature.

Author: dhruvarya-db

docs/source/delta-drop-feature.md

@@ -25,7 +25,7 @@ To remove a Delta table feature, you run an `ALTER TABLE <table-name> DROP FEATU
 You can drop the following Delta table features:
 
 - `deletionVectors`. See [_](delta-deletion-vectors.md).
-- `v2Checkpoint`
+- `v2Checkpoint`. See [V2 Checkpoint Spec](https://github.com/delta-io/delta/blob/master/PROTOCOL.md#v2-spec). Drop support for V2 Checkpoints is available in <Delta> 3.1.0 and above.
 
 You cannot drop other [Delta table features](https://github.com/delta-io/delta/blob/master/PROTOCOL.md#valid-feature-names-in-table-features).