Fix anchors in API doc

It looks like the docs were originally configured to work with GitHub-flavored Markdown easily. Unfortunately, in GFM/Markdown the auto-anchoring is kind of poor, resulting in a lot of broken links in the ToC. This change inserts an anchor with a non-conflicting name at each section and wires up the ToC to point to it.

See https://github.com/eheikes/protractor/blob/api-fix-anchors/docs/api.md for how it works.