update docs

docs/en/guide/markdown.md

@@ -932,6 +932,18 @@ Some content here.
 Some more content here.
 ```
 
+Here, `my-base-section` is the generated id of the heading element. In case it's not easily guessable, you can open the part file in your browser and click on the heading anchor (`#` symbol left to the heading when hovered) to see the id in the URL bar. Or use browser dev tools to inspect the element. Alternatively, you can also specify the id to part file like this:
+
+```md
+## My Base Section {#custom-id}
+```
+
+and include it like this:
+
+```md
+<!--@include: ./parts/basics.md#custom-id-->
+```
+
 ## Math Equations
 
 This is currently opt-in. To enable it, you need to install `markdown-it-mathjax3` and set `markdown.math` to `true` in your config file: