docs: doc internal links are now working & unstable directory has been removed

There is no need to keep an unstable directory since we don't keep other versions.

Links from docs to docs are now working. For example:

>This directive is deprecated. Prefer using the $timeline service. This directive is used to animate an element in conjunction with an **animate** directive

**animate** is now a correct link to the directive animate (inside the docs).
To make sure internal links work, you need to comment internal docs like this:

>This directive is deprecated.  Prefer using the $timeline service.  This element is used to specify the animation of an element in a **{@link api/directive/faAnimation faAnimation}** directive

Should be:

>This directive is deprecated.  Prefer using the $timeline service.  This element is used to specify the animation of an element in a **{@link faAnimation faAnimation}** directive

Don't use the full link for internal doc, the parser will make it for you.