Add subsection anchor to all content

[ibilon]

For now it's only applied to the manual, https://github.com/HaxeFoundation/haxe.org/blob/staging/src/generators/Manual.hx#L211
but the documentation could use it, the blog too.

Also maybe change the style to something more common like https://gfycat.com/PopularPlayfulCrocodile

[markknol]

When you have changed it, let me know, ill apply the same thing to the cookbook for consistency.