Add writui to the Markdown tool directory

[aakashns]

Adds a Markdown Guide tool page for writui, a free terminal writing app for macOS and Linux. The page includes the project icon, an editor screenshot, and a conservative syntax table. It explains that writui keeps Markdown source visible and exports .md; the support statuses describe the editor's own styling and interactions, not a separate HTML renderer.

Disclosure: I created writui. I checked the current v0.17.0 build with synthetic text in an isolated throwaway vault, including headings, emphasis, lists, task markers, tables, links, images, code fences, and raw HTML. I also checked the parser options and the project's public documentation for unsupported features. The five-minute first writing session shows the normal install and workflow.

Validation: the YAML front matter parses, all 28 syntax IDs and status values match this repository's data files, assets exist, and git diff --check passes. I could not run the Jekyll site locally because the locked Bundler version is not installed, so the preview build should confirm the final layout.