Add Search UI to config

[colleenmcginnis]

⚠️ Dependent on elastic/search-ui#1093

Adds a new Search UI book to the config. 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 is dependent on a PR in elastic/search-ui that adds an AsciiDoc index file (Add inital AsciiDoc index page search-ui#1093).
• This PR adds a stub Search UI book with just a single index page from the elastic/search-ui repo. This stub book will not be indexable by search engines.
• After this PR is merged, I will open a PR in elastic/search-ui 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 the elastic/search-ui PR is merged and the full Search UI book is live, I'll open another PR in this repo to remove noindex so the book will start being indexed by search engines.

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