Add clickable headers in documentation

[sbdchd]

Make the documentation headers clickable, so that they are more easily shared.
See flycheck for example.

The headers already have the ids, they just need to be linked.

Edit: changed doc link

[nashamri]

This syl20bnr/develop.spacemacs.org@83b3067 should fix it. But I need to push a PR to spacemacs repo to include the new js file in core/core-documentation.el

[syl20bnr]

I merged the PR in develop branch.

[nashamri]

@syl20bnr This should be labelled Fixed in develop.spacemacs.org

[syl20bnr]

Thank you for the report.
Fixed in develop.spacemacs.org will be merged to spacemacs.org at next major release.