update N-API docs

[richardlau]

• doc,n-api: fix introduced_in metadata
  Node.js v7.10.0 did not contain N-API. Update the introduced_in
  metadata to prevent a broken 7.x "View another version" link in the
  N-API docs.
• doc,n-api: update N-API version matrix for v12.x

The first commit ("fix introduced_in metadata") fixes a 404 not found link
(https://nodejs.org/docs/latest-v7.x/api/n-api.html) which was disovered
in #27267 (https://travis-ci.com/nodejs/node/jobs/193473667#L1277).

As seen in the version matrix, N-API was backported to Node.js 6. I set
introduced_in for v8.0.0 based on Node.js 6 being End-of-Life and
setting to a 6.x release would still result in the broken link to 7.x due to
the simplistic way the "View another version" links are generated:

node/tools/doc/html.js

Lines 417 to 423 in f4572cc

	function isDocInVersion(version) {
	const [versionMajor, versionMinor] = version.num.split('.').map(Number);
	if (docCreatedMajor > versionMajor) return false;
	if (docCreatedMajor < versionMajor) return true;
	if (Number.isNaN(versionMinor)) return true;
	return docCreatedMinor <= versionMinor;
	}

For the version matrix the non-12.x rows of the N-API version 4 column
should be handled in #27567 (raised by a first time contributor).

cc @nodejs/n-api @nodejs/documentation

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

[nodejs-github-bot]

Lite-CI: https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/3548

[BridgeAR]

I wonder if we should add another optional information to skip some versions. We could also automate it by checking the release dates of each version.

[mhdawson]

@BridgeAR I agree being able to skip versions would be good.

[mhdawson]

I hope this does not cause confusion, but with 6.x being EOL I'm willing to give it a try.

[addaleax]

Landed in b3ffd1f 98a552d