Add inital AsciiDoc index page

[colleenmcginnis]

Description

Adds a placeholder AsciiDoc index file to be used in a new Search UI book.

This is part of an effort to move the current Search UI docs to AsciiDoc. There is a bit of a 🐔 / 🥚 problem with this process:

• This PR must be merged before the elastic/docs PR that adds the Search UI book (Add Search UI to config docs#3140).
• The elastic/docs PR adds a stub Search UI book with the index.asciidoc file as the only page. This stub book will not be indexable by search engines.
• After that elastic/docs PR is merged, I will open a PR in this repo to add all the AsciiDoc content. That PR will include a preview that the docs and search teams can use to validate the AsciiDoc version of the Search UI docs.
• After that PR is merged and the full Search UI book is live, I'll open another PR in the elastic/docs repo to remove noindex so the book will start being indexed by search engines.

List of changes

• index.asciidoc: Placeholder index file to set us up for creating AsciiDoc previews of the new Search UI book.
• .gitignore: Adds html_docs, which is a directory that may be generated when you build the docs locally.

Associated Github Issues

Related to elastic/docs#3140