Update zha.markdown to mention recently added group support

[Hedda]

Proposed change

Just add a short mentioning of device grouping support into entities since it has been a highly requested feature popularized by the Philips Hue Bridge (with its "light groups") that has been missing from the ZHA integration component and was recently added and is already available in the current branch.

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

Not done by me but added by @dmulcahey via all these pull requests:

• Add the ability to create new Zigbee groups to the ZHA config panel frontend#4384
• Add group editing to the ZHA config panel frontend#4382
• Add group binding to the ZHA config panel and misc. cleanup frontend#4466
• Add group detail view to the ZHA config panel frontend#4380
• Add Zigbee group removal to the ZHA config panel frontend#4376
• Add Zigbee group viewing to ZHA config panel frontend#4365
• Add group entity support to ZHA core#33196
• Add entities for ZHA groups containing switches core#33207
• Add entities for ZHA fan groups core#33291
• Add ZHA service to issue group commands core#28823
• Add ZHA group API core#29641
• Enhance ZHA group removal core#30183
• Add Zigbee group binding to ZHA core#30433
• Update ZHA group entity when Zigbee group membership changes core#33378
• ZHA group and device cleanup core#31260

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
• The documentation follows the Home Assistant documentation standards.

[frenck]

Thanks, @Hedda 👍