Headings in doc-site are linkable

[andrew-codes]

Make the headings found in the gatsby docs site linkable. Would also be nice to have a copy icon button show on hover that will copy the link to the clipboard for easy pasting in Slack, issues, etc.

In order to maintain compatibility with existing links to headings already found in the documentation, headings' anchors should be the same as the heading in kebab-case (e.g. i-am-kebab-case)