doc: fix autoLink function, dont autolink from headers

[jonchurch]

This PR is for the tooling historically used to generate the HTML for the doc site

Problem

The weirdness we saw in lodash/lodash.com#300 (comment) was related to bugs in the preprocess and postprocess step in this repo when generating the doc site html.

We saw some md links to the source code not being converted into html, and double links in the h3s for some methods (the anchor link, then the h3 content itself wrapped in a link).

Solution

Change 1: the issue was that the preprocess step was only converting the md links that had both a source and an npm package link. The regexp just was too strict, and didnt match on the API surfaces that had a source link but no npm link (bc there is no npm package for things like _.VERSION)

Change 2: exclude headers from the autolinking behavior. The selector there was matching anything in
.doc-container code that looked like a reference to a lodash method. But that also caught several headers.

Context

The process when running npm run doc:sitehtml goes:

• Generate markdown from the JSDoc (using docdown), writes to doc/README.md
• Preprocess said generated markdown in the lib/main/build-site.js script
  • Change 1 of this PR is at this step, converting md links like [source](the github source...) into html links
• Run that markdown through marky-markdown
• Run a postprocess step on the html
  • Change 2 of this PR was here, tweak the autolinking behavior when referencing a lodash method in doc body

[ljharb]

we should definitely convert this to something less "parse regex from html"ish

[falsyvalues]

we should definitely convert this to something less "parse regex from html"ish

And add some smoke tests for this part.

[jonchurch]

just signed my existing commits and force pushed 🔐