tools: adds doc links to the stability index reference

[michaelcox]

Checklist

• make -j4 test (UNIX)
• commit message follows commit guidelines

Affected core subsystem(s)

tools

Description of Change

This modifies the script that generates the docs to create a static link from each Stability Index callout bar to the Stability Index explanations in documentation.html. This was a first-commit request from @Trott. Here is what he pitched to me:

Many APIs have a Stability Index associated with them. Check out https://nodejs.org/dist/latest-v7.x/docs/api/modules.html#modules_modules for an example (the Modules API is Locked, Stability Index 3).
It would be great if there could be a link in the Stability Index banner to the definition of the various Stability Indexes. If I don't know what "Locked" means, then I can click the link and find out.
Modify html.js to include such a link. This will involve some HTML of course, and probably some CSS and maybe some back-and-forth about usability/styling once the PR is open.

I looked for any precedent for styling, and the best I could find is that when features are deprecated, a simple link is put to the new API to use, inside the banner. This similarly just links the first part (e.g. Stability: 2) to the documentation.html page. I'm obviously open to other styling suggestions, but this seemed the most straightforward. There doesn't seem to be any precedent in the docs for other things like a ? icon or similar.

Also I'm just linking to the main anchor for the Stability Index explanations, rather than to a specific one. My thinking here is that there are only a handful of them, and they should all fit nicely above the fold for anyone reading that page. It didn't seem necessary to create additional anchors.

Note that I had to make a couple more variables in the commit here to make the tests pass with the 80 character line limit.

[hiroppy]

I think that the commit message should be tools rather than doc;)

[michaelcox]

Thanks - updated to be tools instead. Makes more sense. :)

[hiroppy]

cc @nodejs/documentation

[Trott]

I don't think this file is exercised by any tests, but it is covered by lint rules, so here's a CI lint run for it: https://ci.nodejs.org/job/node-test-linter/7386/

[thefourtheye]

Nit: I would prefer writing this as,

`<pre class="${classNames}">
    <a href="${docsUrl}">$1 $2</a>$3
 </pre>`

[michaelcox]

I've updated to bring the closing </pre> to the same line, but I can't write it nested the way you have it because of the default formatting of a <pre> tag. Doing it the way you had it adds another 20px-ish left padding into the bar.

[fhinkel]

Thanks. Landed in dd76f5f