doc: Document markup syntax

Not depicted:

- Diagrams using Lua filter and dot
- fenced code attributes (enabled by the attributes extension)

doc/contributing/contributing-to-documentation.chapter.md

@@ -1,6 +1,6 @@
 # Contributing to this documentation {#chap-contributing}
 
-The DocBook sources of the Nixpkgs manual are in the [doc](https://github.com/NixOS/nixpkgs/tree/master/doc) subdirectory of the Nixpkgs repository.
+The sources of the Nixpkgs manual are in the [doc](https://github.com/NixOS/nixpkgs/tree/master/doc) subdirectory of the Nixpkgs repository. The manual is still partially written in DocBook but it is progressively being converted to [Markdown](#sec-contributing-markup).
 
 You can quickly check your edits with `make`:
 
@@ -22,3 +22,73 @@ $ nix-shell
 ```
 
 If the build succeeds, the manual will be in `./result/share/doc/nixpkgs/manual.html`.
+
+## Syntax {#sec-contributing-markup}
+
+As per [RFC 0062](https://github.com/NixOS/rfcs/pull/72), all new documentation content should be written in [CommonMark](https://commonmark.org/) Markdown dialect.
+
+Additionally, the following syntax extensions are currently used:
+
+- []{#ssec-contributing-markup-anchors}
+  Explicitly defined **anchors** on headings, to allow linking to sections. These should be always used, to ensure the anchors can be linked even when the heading text changes, and to prevent conflicts between [automatically assigned identifiers](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/auto_identifiers.md).
+
+  It uses the widely compatible [header attributes](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/attributes.md) syntax:
+
+  ```markdown
+  ## Syntax {#sec-contributing-markup}
+  ```
+
+- []{#ssec-contributing-markup-anchors-inline}
+  **Inline anchors**, which allow linking arbitrary place in the text (e.g. individual list items, sentences…).
+
+  They are defined using a hybrid of the link syntax with the attributes syntax known from headings, called [bracketed spans](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/bracketed_spans.md):
+
+  ```markdown
+  - []{#ssec-gnome-hooks-glib} `glib` setup hook will populate `GSETTINGS_SCHEMAS_PATH` and then `wrapGAppsHook` will prepend it to `XDG_DATA_DIRS`.
+  ```
+
+- []{#ssec-contributing-markup-admonitions}
+  **Admonitions**, set off from the text to bring attention to something.
+
+  It uses pandoc’s [fenced `div`s syntax](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/fenced_divs.md):
+
+  ```markdown
+  ::: {.warning}
+  This is a warning
+  :::
+  ```
+
+  which renders as
+
+  > ::: {.warning}
+  > This is a warning.
+  > :::
+
+  The following are supported:
+
+    - [`caution`](https://tdg.docbook.org/tdg/5.0/caution.html)
+    - [`important`](https://tdg.docbook.org/tdg/5.0/important.html)
+    - [`note`](https://tdg.docbook.org/tdg/5.0/note.html)
+    - [`tip`](https://tdg.docbook.org/tdg/5.0/tip.html)
+    - [`warning`](https://tdg.docbook.org/tdg/5.0/warning.html)
+
+- []{#ssec-contributing-markup-definition-lists}
+  [**Definition lists**](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/definition_lists.md), for defining a group of terms:
+
+  ```markdown
+  pear
+  :   green or yellow bulbous fruit
+
+  watermelon
+  :   green fruit with red flesh
+  ```
+
+  which renders as
+
+  > pear
+  > :   green or yellow bulbous fruit
+  >
+  > watermelon
+  > :   green fruit with red flesh
+
+For contributing to the legacy parts, please see [DocBook: The Definitive Guide](https://tdg.docbook.org/) or the [DocBook rocks! primer](https://web.archive.org/web/20200816233747/https://docbook.rocks/).