Make Headings clickable anchors about circleci-docs

[FelicianoTech]

I'd really hate to switch from Kramdown as that'll be an extra dependency. Kramdown (what is used now) is included by default.

I was thinking of adding the link icon to the right of the header like most doc websites have. What do you think? An example is how Stripe does it: https://stripe.com/docs/checkout/tutorial

But with the chain link icon instead of a literal anchor.

[eric-hu]

The side anchor link works. I think as long as it's automatically generated instead of requiring the markdown writer to insert a link, that goes a long way in utility.