Add permalinks to the doc headings

[nashamri]

This PR is to add permalinks to the documentation headings and sections to make it easy to share links and to fix this website issue syl20bnr/spacemacs.org#13.

[TheBB]

This won't work because the HTML files aren't all at the same depth. :-/

[nashamri]

I can't think of any way to overcome this other than just using the full path to that javascript file. It's already on http://develop.spacemacs.org/js/permalinks.js

Should I just change it to be http://www.spacemacs.org/js/permalinks.js (the stable site)?

[nashamri]

I added the absolute path to that js file. It's not on the stable branch yet but on the develop branch of the website. If this is okay, we can move that file to the stable branch.

[JAremko]

can't think of any way to overcome this other than just using the full path to that javascript file.

@nashamri Can You use the same trick as with the css readtheorg injection ?

https://github.com/nashamri/spacemacs/blob/21cfdf8b040c73fe01ede2dfaec60eb826d472a2/core/core-documentation.el#L152

[syl20bnr]

Thank you ! 👍
For now the absolute path will do it. We can go the injection route later (or when it breaks 😁).
Cherry-picked into develop branch, you can safely delete your branch.