Add 'Global Index' to Features page, under Partitioning

[mjonss]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

Add 'Global index' under Partitioning section of the features page

2. Describe your suggestion or addition.
   It was GA in 8.5 (8.4) and kind of experimental earlier.

3. Provide some reference materials (such as documents and websites) if you could.
   https://docs.pingcap.com/tidb/stable/basic-features/#partitioning
   https://docs.pingcap.com/tidb/stable/partitioned-table/#global-indexes
   https://docs.pingcap.com/tidb/stable/release-8.3.0/
   https://docs.pingcap.com/tidb/stable/release-8.4.0/
   https://docs.pingcap.com/tidb/stable/release-8.5.0/