[HUDI-7025] Consolidate index and functional index config groups

[codope]

Change Logs

Subsume FUNCTIONAL_INDEX config subgroup in the INDEX subgroup and remove INDEXING group.

Impact

All index configs will be listed together.

Risk level (write none, low medium or high below)

none

Documentation Update

Describe any necessary documentation update if there is any new feature, config, or user-facing change. If not, put "none".

• The config description must be updated if new configs are added or the default value of the configs are changed
• Any new feature or user-facing change requires updating the Hudi website. Please create a Jira ticket, attach the
  ticket number here and follow the instruction to make
  changes to the website.

Contributor's checklist

• Read through contributor's guide
• Change Logs and Impact were stated clearly
• Adequate tests were added if applicable
• CI passed

[hudi-bot]

CI report:

• 21d50d7 Azure: SUCCESS

Bot commands

@hudi-bot supports the following commands:

• @hudi-bot run azure re-run the last Azure build