make headings have an accessible anchor

[butonic]

so we can link to headings like with the one below

⛓ Level 1

⛓ Level 2

⛓ Level 3

edit: ... well sort of ...

[xoxys]

Anchors are already available e.g. https://owncloud.github.io/extensions/ocis_reva/users/#json-driver. I know it would be cool to have a link you can easily copy. But it's not that easy with hugo and even harder if you want to avoid JavaScript as much as possible...

[xoxys]

https://github.com/gohugoio/gohugoioTheme/blob/9d6c945dcb655c2b6295c0f5fad23a92b0a9e5f3/assets/js/anchorforid.js

That's how the gohugo theme solved it. I'll have a look later

[xoxys]

Fixed by #4 @butonic please review