doc: do not link in the headings

If there is a link in the headings, when the ToC is generated, that is
not properly linked and the square brackets are left as they are.

Even if we fix this, different parts of the heading will link to
different sections or even different pages. For example,

    ### What makes [`Buffer.allocUnsafe()`] and
          [`Buffer.allocUnsafeSlow()`] "unsafe"?

will point to three different sections. `allocUnsafe` and
`allocUnsafeSlow` will link to their corresponding sections and all
other words actually link to the heading in the document. This could be
visually confusing.

PR-URL: https://github.com/nodejs/node/pull/9416
Fixes: https://github.com/nodejs/node/issues/9331



Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Brian White <mscdex@mscdex.net>