I want to create a link on a page to a heading on another page.
I don't understand how to define the link.
The information at docs.plone.org aren't helping.
The docs also say
"Plone automatically creates Anchors for headings and subheadings"
which I'm not seeing.
Not that I know of. But once an internal anchor is referenced, there is the anchor in the page. As a workaround you could copy the link and use it as an external link. It is not perfect, like you do not get link-integrity checks and if you rename pages or folder the link wont get updated.
Slightly off topic, but if you enable 'Table of Contents' on a page, you will also 'get anchors' (you find it in the 'Settings tab' when editing a page).
It would be possible to 'hide the Table of Contents' , just to have the anchors automatically generated. (I assume the best way would be with CSS, place it 'off screen' (or maybe it can event be hidden with display: none; )
UPDATE: IMPORTANT: If you keep changing the content, this might not be a good idea, since the anchors have IDs (starting from 1), and adding a new heading will result in linking to the wrong anchor.