Convert .md files to .mdx to enable glossary system

[dhtclk]

Problem

The glossary system only works with .mdx files, but most of our docs are .md files.

Currently: 22 .mdx files can use glossary tooltips
Goal: ~400 .md files converted to .mdx for full glossary coverage

Tasks

• Convert .md → .mdx files
• Update internal links and references
• Test that everything still builds and works

Related

Depends on (#4164) (Glossary System)