Add ratatui-markdown. (#320)

* Link to your project (GitHub/crates.io):
https://github.com/celestia-island/ratatui-markdown

* Description (optional): A Rust library providing markdown rendering,
Mermaid diagrams, syntax highlighting, collapsible JSON/TOML tree views,
and a rich hybrid scroll system.

* Screenshot or gif (strongly recommended): 

<img width="2466" height="1200" alt="code-highlight"
src="https://github.com/user-attachments/assets/7c783587-ec6f-48ff-8bbc-9d6123530f28"
/>
<img width="2466" height="1199" alt="mermaid"
src="https://github.com/user-attachments/assets/be90ee9f-c16b-42bc-a33b-d95b2e1dcde0"
/>
<img width="2466" height="1197" alt="tree-list"
src="https://github.com/user-attachments/assets/5d30d83b-e7d3-4a37-b4be-4def7f3fb1b2"
/>
<img width="2466" height="1198" alt="custom-block"
src="https://github.com/user-attachments/assets/b17231b5-be9c-4976-af6b-aed494338a52"
/>
<img width="2466" height="1201" alt="image"
src="https://github.com/user-attachments/assets/384c49ea-3def-4782-ab2e-86ea74221a49"
/>

Author: langyo

README.md

@@ -67,6 +67,7 @@ Aside from those listed here, many other apps and libraries can be easily be fou
 - [ratatui-toaster](https://crates.io/crates/ratatui-toaster) - An extremely lightweight toast engine for ratatui.
 - [ratatui-cheese](https://crates.io/crates/ratatui-cheese) - Bubbletea-inspired widgets for ratatui, including spinner, help, tree, paginator and list.
 - [ratatui-code-editor](https://github.com/vipmax/ratatui-code-editor) - A code editor widget for ratatui, syntax highlighting powered by tree-sitter.
+- [ratatui-markdown](https://github.com/celestia-island/ratatui-markdown) - A Rust library providing markdown rendering, Mermaid diagrams, syntax highlighting, collapsible JSON/TOML tree views, and a rich hybrid scroll system.
 - [term-rustdoc](https://github.com/zjp-CN/term-rustdoc) - A TUI for Rust docs that aims to improve the UX on tree view and generic code.
 - [throbber-widgets-tui](https://crates.io/crates/throbber-widgets-tui) - A widget that displays throbber.
 - [tui-additions](https://crates.io/crates/tui-additions) - Additions to the rust tui crate.