Restore original syntax guide and quick reference

- Restore full syntax/index.md landing page
- Restore original syntax/quick-ref.md with all content
- Fix cumulative-docs links to point to docs-content
- Add TODO comment about keeping quick-refs in sync

Author: leemthompo

docs/syntax/index.md

@@ -1,5 +1,29 @@
 # Syntax guide
 
-:::{note}
-👋 The syntax guide has moved to [elastic.co/docs](https://www.elastic.co/docs/contribute-docs/syntax).
-:::
+Learn about the custom Markdown syntax used in Elastic documentation.
+
+## Quick reference
+
+Refer to the [quick reference](quick-ref.md) for a condensed syntax cheat sheet.
+
+## How it works
+
+Elastic Docs V3 uses a custom implementation of [MyST](https://mystmd.org/) (Markedly Structured Text), which extends standard Markdown with directive syntax.
+
+If you know [Markdown](https://commonmark.org), you already know most of what you need. If not, the CommonMark project offers a [10-minute tutorial](https://commonmark.org/help/). 
+
+When you need more than basic Markdown, you can use [directives](directives.md) to add features like callouts, tabs, and diagrams.
+
+## GitHub Flavored Markdown support
+
+V3 supports some GitHub Flavored Markdown extensions:
+
+**Supported:**
+- Tables (basic pipe syntax)
+- Strikethrough with `~~text~~` (renders as ~~text~~)
+
+**Not supported:**
+- Task lists
+- Automatic URL linking: https://www.elastic.co
+  - Links must use standard Markdown syntax: [Elastic](https://www.elastic.co)
+- Using a subset of HTML