[waveshare_epaper] Add support for 4.26inch display

[nisargjhaveri]

Description:

Add entry for the new display supported with the linked PR.

Pull request in esphome with YAML changes: esphome/esphome#6941

Checklist:

• [x] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [ ] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /index.rst when creating new documents for new components or cookbook.