[DOCS] Add index-extra-title-page.html for direct HTML migration

[jrodewig]

Adds the index-extra-title-page.html file, which will replace index-docinfo.xml and index.x-docinfo.xml once we migrate our docs build to direct HTML.

This file contains the following introductory text for the ES Reference index:

The file is inert until the direct HTML migration happens. Even then, this change is no-op.

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[nik9000]

I think it needs to be index-x-titlepage or it won't render!

[jrodewig]

@nik9000 I tested this locally with --direct-html and the current file name and it appears to render. Not sure how to stage a --direct-html preview, but here's a screenshot:

You can tell it's the direct HTML version because it's missing the auto-generated anchors from the docbook version:

Just to see, I did try changing the file to index-x-titlepage locally, but that appears to break it:

[nik9000]

I think the trick is that in conf.yaml we say we use index.x.asciidoc so you have to do that too to test these.

[debadair]

Thanks for doing this @jrodewig !

[jrodewig]

@nik9000 I attempted renaming this to index.x-extra-title-page.html and it doesn't render.

I believe this line in Util.pm may be stripping out the .x:
https://github.com/elastic/docs/blob/master/lib/ES/Util.pm#L376

    my $extra_title_page = $index->basename;

My guess is that it thinks .x.asciidoc is the extension. Haven't confirmed that though.

[nik9000]

Ah! Well, let's merge it and give it a go!

[nik9000]

My guess is that it thinks .x.asciidoc is the extension. Haven't confirmed that though.

That'd be really good actually.