Add Inkiostro to the tool directory

[mi0772]

Adds a directory entry for Inkiostro, a native Markdown editor for macOS, iPadOS, and iOS, with a separate Android version.

• _tools/inkiostro.md — the entry and its syntax support table
• assets/images/tool-icons/inkiostro.png — 256×256, to match the other icons
• assets/images/tools/inkiostro-editor.png — screenshot of the editor and the live preview

I'm the developer of the application. Following the contribution notes I've kept the description to what it does, listed the elements it doesn't support, and included a quirk users run into.

On the support table: the preview is rendered with markdown-it 14.1.0 — CommonMark plus tables, strikethrough, footnotes, task lists, and heading anchors — so I filled the table from the renderer's actual configuration, then verified the ambiguous rows by running test input through the exact bundle the application ships rather than going from memory. Two are worth flagging:

• Heading IDs — Partial. IDs are generated automatically from the heading text and drive the document outline, but the {#custom-id} syntax isn't supported: it stays in the heading as literal text. It's mentioned in the prose as well.
• Definition lists, emoji shortcodes, highlight, subscript, superscript — No. Each needs a markdown-it plugin the application doesn't load.

Happy to reword anything that reads as promotional, or to cut the two subsections at the end if they're more detail than the directory wants.