Style guide: HTML anchors

wiki/src/contribute/how/documentation/style_guide.mdwn

 [[!meta title="Documentation style guide"]]
 
+- **anchors** (HTML anchors)
+
+  Use HTML anchors to provide shortcuts when pointing people to sections
+  inside a page.
+
+  - *For example*:
+
+    - `<a id="2014">` in `doc/about/finances` to be able to point to
+      `https://tails.boum.org/finances#2014`.
+
+  - Keep them as short as possible as they appear in the URL.
+
+  - Use hyphens instead of underscores to separate words.
+
 - **bulleted lists**
 
   Refer to this article from NN/g on [presenting bulleted