[Docs] Add "Community Themes" section to README

[Star-Trowa]

Problem Statement

Hi LeanBitLab team!

LeanType's theming system is one of its standout features, and a
small community of theme creators has formed around it. Would you
consider adding a "Community Themes" section to the README to help
users discover them?

Proposed Solution

Proposed addition:

Community Themes

• LeanType-themes
  • Curated 16 themes collection (25+⭐, actively maintained by me)

Happy to open a PR with this change if you're open to it. Let me
know if there are specific criteria themes should meet to be listed
(e.g., minimum stars, license, screenshots, activity).

Love using your keyboard!

PS: Here are few screenshots from LeanType-themes to get you interested :)

Alternatives

No response