Add Obsidian Markdown Cheat Sheet guide

[waydelyle]

Adds a new guide note, Obsidian Markdown Cheat Sheet, under 04 - Guides, Workflows, & Courses/Guides, and links it from the [[for Beginners]] MOC.

• Covers core Obsidian Markdown syntax — basics, wikilinks, embeds, callouts, properties, tags, highlight, strikethrough, comments, footnotes, tables, math, and Mermaid — plus a CommonMark/GFM-vs-Obsidian compatibility table.
• Every section cites the official Obsidian Help page it was verified against, with the verification date.
• Renders entirely with built-in Obsidian features; no plugins required.
• Uses the T - How to template frontmatter and the standard Hub footer. A Source section at the end links the maintained original at markdowntools.io, where the same content is available as a vault-ready download and a printable PDF.