Add Markdown Tools to the tool directory

[waydelyle]

Adds a tool page for Markdown Tools, a free online Markdown editor (category: online editor).

• The syntax table is filled in from the application's parser configuration: marked with GFM and line breaks enabled, plus highlight.js, KaTeX math, Mermaid diagrams, GitHub-style callouts, footnotes, and auto-generated heading slugs. Happy to adjust any cell after testing with the markdown-test README.
• Icon included at assets/images/tool-icons/markdowntools.png. No screenshot, per the wiki's optional note — glad to add one if you'd like.
• Description kept short and neutral per the contribution guidelines.